To reproduce:
- insert a fragmet from another RichtextEditor like this:
private void Button_Click(object sender, RoutedEventArgs e)
{
radRichTextBox.Document.Sections.Clear();
radRichTextBox.Document.Sections.Add(new Section());

radRichTextBox.Document.CaretPosition.MoveToLastPositionInDocument(); 
radRichTextBox.DocumentEditor.InsertParagraph();
radRichTextBox.DocumentEditor.InsertFragment(new DocumentFragment(radRichTextEditor1.Document));
radRichTextBox.DocumentEditor.InsertParagraph();
}

Workaround:
There is no point of resetting the section in such way. Nevertheless, you can avoid the exception by using one of the following methods:

radRichTextBox.Document.MeasureAndArrangeInDefaultSize();
radRichTextBox.UpdateEditorLayout();

Local properties are not exported to HTML when style is applied over document element (paragraphs, spans, tables) in case the HtmlExportSettings.StyleExportMode is set to Inline.

To reproduce:
- Add two hyperlinks with differents font and font weights.
- Export them with the following code:
private void button1_Click(object sender, EventArgs e)
{
    HtmlFormatProvider html_provider = default(HtmlFormatProvider);
    string htmlBody = null;
    html_provider = new HtmlFormatProvider();
    html_provider.ExportSettings.StylesExportMode = StylesExportMode.Inline;
    html_provider.ExportSettings.StyleRepositoryExportMode = StyleRepositoryExportMode.ExportStylesAsCssClasses;
    htmlBody = html_provider.Export(radRichTextEditor1.Document);
    File.WriteAllText("test.html", htmlBody);
}
- Open the file and you will notice that the custom styles are not exported (the links are having the same style).

Workaround: 
html_provider.ExportSettings.StylesExportMode = StylesExportMode.Classes;

1. Open RadRichTexEditor and type the following text: "שדג:3"  or open the attached file
2. Save the document as docx file
3. Compare the results in MS Word and in RadRichTexEditor

Observed result: in RadRichTexEditor the text is visualized as: ":3שדג"
Expected result: The text should be visualized as: "שדג:3"

To reproduce:
- Copy the content from the attached document - do not copy the last row.
- Remove the bookmarks like this:

bookmarks = radRichTextEditor1.Document.GetAllBookmarks().ToArray();
foreach (BookmarkRangeStart item in bookmarks)
{
    radRichTextEditor1.Document.GoToBookmark(item);
    radRichTextEditor1.DocumentEditor.DeleteBookmark(item);
   
}

Workaround:
                    var invalidStarts = this.radRichTextBox.Document
                    .EnumerateChildrenOfType<BookmarkRangeStart>()
                    .Where(start => start.End == null)
                    .ToList();

                    foreach (var invalidStart in invalidStarts)
                    {
                        invalidStart.Parent.Children.Remove(invalidStart);
                    } 
Note that when this workaround is used the document history will be deleted as well, which means that the undo operation will no longer hold information for previous operations

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).

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);
    }
}

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;
        }

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.

Workaround:
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.AdjustPaddings(0);
        this.AdjustPaddings(3);
    }

    private void AdjustPaddings(int index)
    {
        RibbonTab tab = ((RibbonTab)this.richTextEditorRibbonBar1.CommandTabs[0]);
        RadRibbonBarGroup group = (RadRibbonBarGroup)tab.Items[index];
        foreach (RadElement el in group.Items)
        {
            el.Padding = new Padding(3, 1, 3, 1);
        }
    }
}