To reproduce:
1 Please change the font via selection list first.
2 Set text cursor to font input field and select the whole text (if not already done).
3 Press backspace or delete key.
4 An error message appears.

Workaround:
protected override void DropDownListFont_SelectedIndexChanged(object sender, PositionChangedEventArgs e)
{
    var ddl = sender as RadDropDownListElement;
    if (ddl.SelectedItem != null)
    {
        base.DropDownListFont_SelectedIndexChanged(sender, e);
    }
}

To reproduce:
- Open a document that contains symbols with the 2015 Q3 SP1 version.
- Export the document using the docx format.
- Open it again.

Workaround:
private void radRichTextEditor1_DocumentChanged(object sender, EventArgs e)
{
    this.SelectAllMatches("");
    this.radRichTextEditor1.ChangeFontFamily(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Symbol"));
    this.SelectAllMatches("");
    this.radRichTextEditor1.ChangeFontFamily(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Symbol"));
}
 
private void SelectAllMatches(string toSearch)
{
    this.radRichTextEditor1.Document.Selection.Clear();
    DocumentTextSearch search = new DocumentTextSearch(this.radRichTextEditor1.Document);
    foreach (var textRange in search.FindAll(toSearch))
    {
        this.radRichTextEditor1.Document.Selection.AddSelectionStart(textRange.StartPosition);
        this.radRichTextEditor1.Document.Selection.AddSelectionEnd(textRange.EndPosition);
    }
}

Support for math type equations. 

If there is sequence with more than one font info which is not declared in a separate group, they all are concatenated and recorded in the imported fonts as a single one with id from the last one. Here is such a problematic font table group:

{\\fonttbl\\f0\\froman\\fcharset0 Times New Roman;\\f1\\froman\\fcharset0 Times New Roman;\\f2\\froman\\fcharset0 Times New Roman;\\f3\\froman\\fcharset0 Times New Roman;\\f4\\froman\\fcharset0 Times New Roman;\\f5\\froman\\fcharset0 Times New Roman;}

The scenario is very common, as in MS Word the option Table -> Layout -> Cell Size -> AutoFit ->  "AutoFit Contents" changes the PreferredWidth of the table and its columns to Auto.
Also the tables imported from HTML have these properties set by default.

Scenarios:
- Auto-sized table occupies minimal space.
- All auto-sized columns in a table (table itself could be, or could be not, auto-sized) should always be sized proportionally to their content length. Currently, such columns are sized equally in the case when there is enough space for all of the content, which is unexpected. (also see the attached images for the scenario with fixed-width table + auto-sized columns).

To reproduce:
- Export document with an inline UI elements to HTML.
- Import the document.

 Workaround:
- Manually export the UI elements information and then create new UI elements when the document is imported:
void ImportSettings_InlineUIContainerImporting(object sender, InlineUIContainerImportingEventArgs e)
{
    PropertyInfo property = typeof(InlineUIContainerImportingEventArgs).GetProperty("Handled");
    property.GetSetMethod(true).Invoke(e, new object[] { true });

    ObjectHandle handle = Activator.CreateInstance("Telerik.WinControls.UI", e.CommentContent);
    Object control = handle.Unwrap();
    InlineUIContainer container = new InlineUIContainer();
    RadElementUIContainer radContainer = new RadElementUIContainer(control as RadElement);
    container.UiElement = radContainer;
    container.Height = 30;
    container.Width = 300;
    e.TargetParagraph.Inlines.Add(container);
}
//export
private void btnWrite_Click(object sender, EventArgs e)
{
    HtmlFormatProvider provider = new HtmlFormatProvider();
    provider.ExportSettings.InlineUIContainerExporting += ExportSettings_InlineUIContainerExporting;         
  
    File.WriteAllText(Environment.GetFolderPath(Environment.SpecialFolder.Desktop) + "\\TelerikUITest.html", provider.Export(this.radRichTextEditor1.Document));
}

void ExportSettings_InlineUIContainerExporting(object sender, Telerik.WinForms.Documents.FormatProviders.Html.InlineUIContainerExportingEventArgs e)
{
    string control = ((RadElementUIContainer)e.InlineUIContainer.UiElement).Element.ToString();
    e.CommentContent = control;
}

To reproduce:
- Add some text and change its forecolor.
- Add more text and change its highlighting
- Save as rtf and open it in WordPad.

Workaround:
void radRichTextEditor1_CommandExecuting(object sender, CommandExecutingEventArgs e)
{
    if (e.Command is PasteCommand)
    {
        e.Cancel = true;
        PasteNewText();
    }
}

public void PasteNewText()
{
    DocumentFragment clipboardDocument = null;
    string clipboardText = null;
    bool clipboardContainsData = false;

    if (ClipboardEx.ContainsDocument(null))
    {
        clipboardDocument = ClipboardEx.GetDocument();
        clipboardContainsData = true;
    }
    else if (ClipboardEx.ContainsText(null))
    {
        clipboardText = ClipboardEx.GetText(null);
        clipboardContainsData = true;
    }

    if (!clipboardContainsData)
    {
        return;
    }

    if (clipboardDocument != null)
    {
        RadDocument doc = new RadDocument();
        RadDocumentEditor editor = new RadDocumentEditor(doc);
        editor.InsertFragment(clipboardDocument);
         
        TxtFormatProvider provider = new TxtFormatProvider();
        string plainText = provider.Export(doc);

        this.radRichTextEditor1.RichTextBoxElement.ActiveDocumentEditor.Insert(plainText);
    }
    else if (!string.IsNullOrEmpty(clipboardText))
    {
        this.radRichTextEditor1.RichTextBoxElement.ActiveDocumentEditor.Insert(clipboardText);
    }
}

Workaround: use different instances of RibbonUI

Workaround:

private void CopyButton_Click(object sender, EventArgs e)
{
    this.radRichTextEditor1.Document.EnsureDocumentMeasuredAndArranged();
    if (this.radRichTextEditor1.Document.Selection.IsEmpty)
    {
        return;
    }

    string selectedText = this.radRichTextEditor1.Document.Selection.GetSelectedText();
    DocumentFragment fragmentToCopy = this.radRichTextEditor1.Document.Selection.CopySelectedDocumentElements(true);
    DataObject dataObject = new DataObject();
    if (selectedText != "")
    {
        ClipboardEx.SetText(null, selectedText, dataObject);
    }

    ClipboardEx.SetDocument(fragmentToCopy, dataObject);
    ClipboardEx.SetDataObject(dataObject);
}

Note: this problem is related to the following feedback item: http://feedback.telerik.com/Project/154/Feedback/Details/174882-fix-radrichtexteditor-spellcheckingdialog-should-not-be-closed-when-a-misspell 
You should apply the suggested workaround in order to be able to replicate the problem.

Steps to reproduce:
1. Enter some misspelled words.
2. Open the spell checking form by using the context menu.
3. "Ignore" the first misspelled word. 
4. The next wrong word is highlighted. If you add it to the dictionary, the previously ignored word is highlighted as well.

Workaround: use the "Ignore All" option.

How to reproduce:
public Form1()
        {
            InitializeComponent();

            richTextEditorRibbonBar1.AssociatedRichTextEditor = radRichTextEditor1;
            radRichTextEditor1.Insert("This is a example for the \"ContextMenu shows up even when disabled\" error. If you right click on a word nothing happens (just as it is supposed to be) now klick \"Find Next Error\" and there it is (not supposed to be)\n\nlkjds klsdjfio jlk sdjfi lsdifuioew rlsoidf  sjdiuf oipds jifpodsuf ");

            radRichTextEditor1.IsContextMenuEnabled = false;
        }

        private void dropdown_PopupOpening(object sender, CancelEventArgs args)
        {
            args.Cancel = true;
        }

Workaround: 
public Form1()
        {
            InitializeComponent();

            richTextEditorRibbonBar1.AssociatedRichTextEditor = radRichTextEditor1;
            radRichTextEditor1.Insert("This is a example for the \"ContextMenu shows up even when disabled\" error. If you right click on a word nothing happens (just as it is supposed to be) now klick \"Find Next Error\" and there it is (not supposed to be)\n\nlkjds klsdjfio jlk sdjfi lsdifuioew rlsoidf  sjdiuf oipds jifpodsuf ");

            radRichTextEditor1.IsContextMenuEnabled = false; 

            FieldInfo fi = this.radRichTextEditor1.RichTextBoxElement.ContextMenu.GetType().GetField("radDropDownMenu", BindingFlags.Instance | BindingFlags.NonPublic);
            RadDropDownMenu dropdown = fi.GetValue(this.radRichTextEditor1.RichTextBoxElement.ContextMenu) as RadDropDownMenu;
            if (dropdown != null)
            {
                dropdown.PopupOpening += dropdown_PopupOpening;
            }
        }

        private void dropdown_PopupOpening(object sender, CancelEventArgs args)
        {
            args.Cancel = true;
        }

Note: it should be closed automatically when all words are corrected.

To reproduce:

1. Enter some misspelled words and open the context menu with right mouse click.
2. Show the SpellCheckingDialog.
3. Add a word to the dictionary. The SpellCheckingDialog will be closed immediately. However, if you press to ignore the word/words, the dialog remains opened.

Workaround: cancel the SpellCheckingDialog.FormClosing event except when the close button is clicked:
public Form1()
{
    InitializeComponent();
  
    this.radRichTextEditor1.IsSpellCheckingEnabled = true;
    RadButton buttonClose = ((SpellCheckingDialog)this.radRichTextEditor1.RichTextBoxElement.SpellCheckingDialog).Controls["buttonClose"] as RadButton;

    buttonClose.MouseDown += buttonClose_MouseDown;
    ((SpellCheckingDialog)this.radRichTextEditor1.RichTextBoxElement.SpellCheckingDialog).FormClosing += SpellCheckingDialog_FormClosing;
}

bool shouldClose = false;

private void buttonClose_MouseDown(object sender, MouseEventArgs e)
{
    shouldClose = true;
}


private void SpellCheckingDialog_FormClosing(object sender, FormClosingEventArgs e)
{
    if (e.CloseReason != CloseReason.FormOwnerClosing)
    {
        e.Cancel = !shouldClose;
        shouldClose = false;
    }
}

To reproduce:
- Write some text.
- Change the current user.
- Repeat three or four times.
- Open spellchecking dialog.

Workaround: handle the CommandExecuted event and manually adjust the location of these forms
public Form1()
{
    InitializeComponent();

    this.radRichTextEditor1.CommandExecuted += radRichTextEditor1_CommandExecuted;
}

private void radRichTextEditor1_CommandExecuted(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutedEventArgs e)
{
    if (e.Command is ShowInsertSymbolWindowCommand)
    {
        ((InsertSymbolDialog)this.radRichTextEditor1.RichTextBoxElement.InsertSymbolWindow).Location = Screen.FromControl(this).WorkingArea.Location;
    }

    else if (e.Command is ShowFindReplaceDialogCommand)
    {
        ((FindReplaceDialog)this.radRichTextEditor1.RichTextBoxElement.FindReplaceDialog).Location = Screen.FromControl(this).WorkingArea.Location;
    }
}

If I drag&drop a RadRichTextEditor i want a wizard like the RichTextBox-WPF-Control. I don't like the ribbon style, because I need a compact editor. So I like the CommandBarStrip look of the "command bar ui"-demo or the ASP.NET Editor.