Workaround: 

Shift + Tab navigation causes drop down opening of RadMultiColumnComboBox

Workaround: 

protected override void OnShown(EventArgs e)
{
    base.OnShown(e);

    LightVisualElement lve = this.radMultiColumnComboBox1.MultiColumnComboBoxElement.TextboxContentElement;
    RadArrowButtonElement arrow = this.radMultiColumnComboBox1.MultiColumnComboBoxElement.ArrowButton;
    lve.MaxSize = new Size(lve.Size.Width - arrow.Size.Width - (int)arrow.Border.Width, 0);
}

Use attached to reproduce!

Workaround 
The column name must match the field name.

To reproduce: 
- Set the DropDownStyle to DropDownList

The RadMultiColumnComboBox throws exceptions if its data source is empty and the AutoSizeDropDownToBestFit property is set to true.

To reproduce: 
- When the filter is set to contains the item is different than the item is selected from the drop-down and the text is taken from the append functionality.

Workaround
Either change the filter to StarrtsWith or remove the suggest functionality. 

When auto-filtering is applied to RadMultiColumnComboBox and mouse double click is performed over a row, the selected row is changed to another row.

Current row is selected in RadMultiColumnComboBox when the mouse's left button is released.

To reproduce: please refer to the attached sample project and try to enter some numeric value or clear the existing one. Then open the popup.

Workaround: use the custom filtering that RadGridView offers http://docs.telerik.com/devtools/winforms/gridview/filtering/custom-filtering

this.radMultiColumnComboBox1.AutoFilter = true;
FilterDescriptor filter = new FilterDescriptor();
filter.PropertyName = this.radMultiColumnComboBox1.DisplayMember;
filter.Operator = FilterOperator.Contains;
this.radMultiColumnComboBox1.EditorControl.MasterTemplate.FilterDescriptors.Add(filter);
this.radMultiColumnComboBox1.EditorControl.EnableCustomFiltering = true;
this.radMultiColumnComboBox1.EditorControl.CustomFiltering += EditorControl_CustomFiltering;

string search = string.Empty;

private void EditorControl_CustomFiltering(object sender, Telerik.WinControls.UI.GridViewCustomFilteringEventArgs e)
{
    search = this.radMultiColumnComboBox1.MultiColumnComboBoxElement.EditorElement.Text;
    if (search != string.Empty)
    {
        e.Handled = true;
        e.Visible = e.Row.Cells[this.radMultiColumnComboBox1.DisplayMember].Value.ToString().Contains(search);
    }
    else
    {
        e.Handled = false;
    }
}

Steps to reproduce:
1. Bind a RMCCB to some data source
2. Enable auto filtering
3. Start the app and enter a value in the text editor of RMCCB
4. Press Tab. You will see that the focus is moved away from the RMCCB but the selection is not changed. Also if you try to open the drop down you will see that it has wrong size. Hitting Escape or Enter instead of Tab produces the correct behavior.

If value member's column is not included in EditorControl (the grid) SelectedValue property always returns null.

When you set the DropDownHeight property to a big number that exceeds the screen's height, the applied values is SystemInformation.WorkingArea.Height - 100 . However, it should be only SystemInformation.WorkingArea.Height. These 100 pixels shouldn't be lost.

NOTE: the same behavior is observed with DropDownWidth as well.

Workaround:

Public Class CustomMultiColumn
Inherits RadMultiColumnComboBox

    Protected Overrides Function CreateMultiColumnComboBoxElement() As RadMultiColumnComboBoxElement
        Return New CustomRadMultiColumnComboBoxElement()
    End Function

    Public Overrides Property ThemeClassName As String
        Get
            Return GetType(RadMultiColumnComboBox).FullName
        End Get
        Set(value As String)
            MyBase.ThemeClassName = value
        End Set
    End Property
End Class

Public Class CustomRadMultiColumnComboBoxElement
Inherits RadMultiColumnComboBoxElement
    Protected Overrides Function GetPopupSize(popup As RadPopupControlBase, measure As Boolean) As Drawing.Size
        Dim s As Size = MyBase.GetPopupSize(popup, measure)
        Return New Size(s.Width, Math.Min(SystemInformation.WorkingArea.Height, Me.DropDownHeight))
    End Function

    Protected Overrides ReadOnly Property ThemeEffectiveType As Type
        Get
            Return GetType(RadMultiColumnComboBoxElement)
        End Get
    End Property
End Class

The RadMultiColumnComboBox control does not update its selection when the user types text in the text field.
The expected behavior is that when the user types text and there is a databound item that matches the input, it should be selected. Otherwise, the current selection in the list should be cleared.

Steps to reproduce:
1. Select the radMultiColumnsComboBox1 at Design time
2. Press F4 for showing Properties
3. Open Columns editor (GridViewDataColumn Collection Editor)
4. Try to add a column
Error Message: Cannot create an instance of the abstract class or interface 'Telerik.WinControls.UI.GridViewDataColumn' because it is an abstract class.

Set the AnimationEnabled property of RadCalendar and DropDownAnimationEnabled property in RadMultiColumnComboBox to false and there will be no effect.

To reproduce:
- Set the AutoFileter property to true.
- Add filter descriptor;
- Start the application and type something in the control - you just need to filter the values so more than one entry is available in the drop down.
- Press Tab or click on other control.
- You will notice that the selected value is set, but the text is not synchronized.

Workaround:
void radMultiColumnComboBox1_Validated(object sender, EventArgs e)
{
    if (radMultiColumnComboBox1.SelectedValue != null)
    {
        string text = radMultiColumnComboBox1.EditorControl.CurrentRow.Cells[radMultiColumnComboBox1.DisplayMember].Value.ToString();
        if (text != radMultiColumnComboBox1.Text)
        {
            radMultiColumnComboBox1.Text = text;
        }
    }
}

To reproduce:
- Add columns to  the control manually.
- Bind the control. Make sure that the FieldNames are different from the column names.
- Start the application and select a value. Retrieve he value upon button click.

Workaround:
Use equal names.

Workaround:
Cancel CurrentRowChanging event if Text is empty:
        this.radMultiColumnComboBox1.EditorControl.CurrentRowChanging += EditorControl_CurrentRowChanging;
        void EditorControl_CurrentRowChanging(object sender, Telerik.WinControls.UI.CurrentRowChangingEventArgs e)
        {
            e.Cancel = string.IsNullOrEmpty(this.radMultiColumnComboBox1.Text) && this.radMultiColumnComboBox1.MultiColumnComboBoxElement.ArrowButton.IsPressed;
        }

To reproduce:
void Form1_Load(object sender, EventArgs e)
{
    this.radMultiColumnComboBox2.AutoSizeDropDownToBestFit = true;

    RadMultiColumnComboBoxElement multiColumnComboElement2 = this.radMultiColumnComboBox2.MultiColumnComboBoxElement;

    multiColumnComboElement2.EditorControl.MasterTemplate.AutoGenerateColumns = true;

    DataTable dt = GetDataTable();
    this.radMultiColumnComboBox2.DataSource = dt;

    FilterDescriptor descriptor3 = new FilterDescriptor("Station", FilterOperator.Contains, null);
    this.radMultiColumnComboBox2.EditorControl.FilterDescriptors.Add(descriptor3);

    FilterDescriptor descriptor4 = new FilterDescriptor("StationName", FilterOperator.Contains, null);
    this.radMultiColumnComboBox2.EditorControl.FilterDescriptors.Add(descriptor4);

    this.radMultiColumnComboBox2.EditorControl.FilterDescriptors.LogicalOperator = FilterLogicalOperator.Or;

    radMultiColumnComboBox2.AutoFilter = true;
}

private DataTable GetDataTable()
{
    DataTable dt = new DataTable();

    dt.Columns.Add("Station"); //, typeof(Int32));
    dt.Columns.Add("StationName");

    System.Data.DataRow row1 = dt.NewRow();
    System.Data.DataRow row2 = dt.NewRow();
    System.Data.DataRow row3 = dt.NewRow();
    System.Data.DataRow row4 = dt.NewRow();
    System.Data.DataRow row5 = dt.NewRow();
    System.Data.DataRow row6 = dt.NewRow();
    System.Data.DataRow row7 = dt.NewRow();
    System.Data.DataRow row8 = dt.NewRow();
    System.Data.DataRow row9 = dt.NewRow();
    System.Data.DataRow row10 = dt.NewRow();

    row1["Station"] = "285";
    row1["StationName"] = "Bob";

    row2["Station"] = "274";
    row2["StationName"] = "Mary";

    row3["Station"] = "222";
    row3["StationName"] = "Joan";

    row4["Station"] = "289";
    row4["StationName"] = "William";

    row5["Station"] = "385";
    row5["StationName"] = "Bob";

    row6["Station"] = "374";
    row6["StationName"] = "Mary";

    row7["Station"] = "331";
    row7["StationName"] = "Jane";

    row8["Station"] = "389";
    row8["StationName"] = "William";

    row9["Station"] = "281";
    row9["StationName"] = "Bob";

    row10["Station"] = "273";
    row10["StationName"] = "Mary";

    dt.Rows.Add(row1);
    dt.Rows.Add(row2);
    dt.Rows.Add(row3);
    dt.Rows.Add(row4);
    dt.Rows.Add(row5);
    dt.Rows.Add(row6);
    dt.Rows.Add(row7);
    dt.Rows.Add(row8);
    dt.Rows.Add(row9);
    dt.Rows.Add(row10);

    return dt;
}

Start the application and type 2738 (273 is the last row), then press the up arrow twice, exception should occur.

Workaround:
public class MyRadMultiColumnComboBox : RadMultiColumnComboBox
{
    protected override RadMultiColumnComboBoxElement CreateMultiColumnComboBoxElement()
    {
        return new MyRadMultiColumnComboBoxElement();
    }
}

public class MyRadMultiColumnComboBoxElement : RadMultiColumnComboBoxElement
{
    protected override void ProcessKeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.Up)
        {
            GridViewRowInfo row = this.GetCurrentRow(true);
            if (row != null)
            {
                int index = row.Index;
                if (index == -1)
                {
                    if (this.Rows.Count > 0)
                    {
                        this.EditorControl.CurrentRow = this.Rows[0];
                    }

                    return;
                }
            }
        }

        base.ProcessKeyDown(sender, e);
    }

    protected override Type ThemeEffectiveType
    {
        get
        {
           return typeof(RadMultiColumnComboBoxElement);
        }
    }
}