The issue is reproducible with Q1 2015 SP1 version. With Q1 2015 it is not.
In order to reproduce it, add at least one empty cell. When you try opening the excel file you get the an error that Excel will try to recover the file.

Should be resolved with LIB version 2015.1.413.

 Introducing an overload of ExportAsync that exposes some "export finished callback" that could be a custom Action delegate. 

When RadGridView is grouped by a column with ShowColumnWhenGrouped property set to False and GroupDescriptors are cleared, the column does not get visible. 

The sort index will not be displayed when applying a custom sorting without adding a SortDescriptor, You can consider implementing Custom Sorting using the IComparable interface instead. As an example please refer to the Custom Sorting with IComparable WPF Demo. Using a generic SortDescriptor would also be an option. 
Please check our online documentation for a further reference: http://docs.telerik.com/devtools/wpf/controls/radgridview/sorting/custom

The problem occurs from 2014 Q2.  It should be resolved with lib version 2014.3.1319.

It would be very nice to have a "transpose" functionality in RadGridView. There are ways to transpose data instead of GridView, but this way it would be difficult to configure bindings, styles and other parameters. PivotGrid isn't suitable in this scenario because of lack of editing.

There is an example of what is needed: https://rotatedwpfdatagrid.codeplex.com/. I was successfully using this example with a standard WPF DataGrid, but I don't know how to accomplish this in RadGridView.

There are already a few threads about this feature:
http://www.telerik.com/forums/transposing-columns-and-rows
http://www.telerik.com/forums/radgridview-can-be-transposed
http://www.telerik.com/forums/displaying-grid-data-in-column-row-format
http://www.telerik.com/forums/reverse-column-and-row-in-the-grid

Thank you!

The problem was initially reproduced by TestStudio users. It can also be reproduced with a codedUI test where the editor is searched by its AutomationID.

Error Message:

Microsoft.VisualStudio.TestTools.UITest.Extension.UITestControlNotFoundException: The playback failed to find the control with the given search properties. Additional Details: 
TechnologyName:  'UIA'
ControlType:  'Text'
AutomationId:  'CellElement_2_1'