Steps to reproduce - by using the code below follow these steps
1) Enter some text in the first cell
2) In the first ComboBox cell ("city") type the beginning of the word, for example "L", then the dataSource gets filtered and contains only items begins with "L". 
3) Move down to the second Item by the down arrow key and choose it by pressing tab key 
4) Now the next cell becomes focused, type in capital "M"
Result: Exception: IndexOutOfRangeException - "Index was outside the bounds of the array."

 DataTable dt = new DataTable();
            dt.Columns.Add("name");
            dt.Columns.Add("city", typeof(int));
            dt.Columns.Add("role", typeof(int));
            radGridView1.DataSource = dt;.

            radGridView1.Columns.RemoveAt(1);
            radGridView1.Columns.RemoveAt(1);

            DataTable dtCities = new DataTable();
            dtCities.Columns.Add("id", typeof(int));
            dtCities.Columns.Add("name");
            dtCities.Rows.Add(1, "New York");
            dtCities.Rows.Add(2, "London");
            dtCities.Rows.Add(2, "Lisabon");

            GridViewComboBoxColumn cityCol = new GridViewComboBoxColumn("city");
            cityCol.DataSource = dtCities;
            cityCol.ValueMember = "id";
            cityCol.DisplayMember = "name";
            cityCol.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            cityCol.DropDownStyle = Telerik.WinControls.RadDropDownStyle.DropDown;
            radGridView1.Columns.Add(cityCol);

            DataTable dtRoles = new DataTable();
            dtRoles.Columns.Add("id", typeof(int));
            dtRoles.Columns.Add("name");
            dtRoles.Rows.Add(1, "Sales Man");
            dtRoles.Rows.Add(2, "Manager");

            GridViewComboBoxColumn roleCol = new GridViewComboBoxColumn("role");
            roleCol.DataSource = dtRoles;
            roleCol.ValueMember = "id";
            roleCol.DisplayMember = "name";
            roleCol.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            roleCol.DropDownStyle = Telerik.WinControls.RadDropDownStyle.DropDown;
            radGridView1.Columns.Add(roleCol);

WORKAROUND: Replace the editor with a new one to avoid reusage
 void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
        {
            if (e.EditorType == typeof(RadDropDownListEditor))
            {
                e.Editor = new RadDropDownListEditor();
            }
        }

Implement a property such as EncloseDataWithQuotes, which should get or set whether the exported data should contain quotes 
http://www.telerik.com/help/aspnet-ajax/p_telerik_web_ui_gridcsvsettings_enclosedatawithquotes.html

To reproduce:
- Create a GridViewMaskBoxColumn with standard mask type and mask set to "aaaaaaaaaa".
- Bind the grid to the following DataTable:
private static DataTable GridDataTable()
{ 
    DataRow dr = default(DataRow);
    DataTable dt = new DataTable();
    dt.Columns.Add("Description");
    
    dr = dt.NewRow();
    dr[0] = "1234567890";
    dt.Rows.Add(dr);
    
    dr = dt.NewRow();
    dr[0] = "abc";
    dt.Rows.Add(dr);
    
    return dt;
}

- Click the first cell in order to enter in edit mode.
- Click the second cell and you will notice that the text is wrong.

Workaround:
- change the value in the CellEditor intialized event handler:

void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    if (e.ActiveEditor is RadMaskedEditBoxEditor)
    {
        RadMaskedEditBoxEditor editor = e.ActiveEditor as RadMaskedEditBoxEditor;
        editor.MaskTextBox.Value = "___________";
        editor.MaskTextBox.Value = (string)radGridView1.CurrentCell.Value;
   
    }
}

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