The sheet selector should hide sheets when their visibility is set to hidden or very hidden. The context menu of the sheet selector should contain Hide and Unhide options that allow to hide a sheet or unhide one of the hidden sheets.

The data validation error message box has an invalid size when shown multiple times

DataValidation of a list with defined names is not taking the source into account. As a result, values are not displayed as options in the cell drop-down list.

Defined names do not appear in the Name Box.

A filter cannot be applied to the RadSpreadsheet when a sheet is protected even though the Filter option in the protection dialog is checked. 

InvalidOperationException (Sequence contains no matching element) when editing a note.

If a worksheet spans more than one page, you can print row and column headings or labels (also called print titles) on every page to ensure that the data is properly labeled.
Duplicate of SpreadProcessing: Repeat specific rows or columns on every printed page (telerik.com)

If the Spreadsheet ribbon has a custom template which hides the Style Gallery, when the Styles collection is edited, a NullReferenceException is thrown. This is because the change causes changes in the Style Gallery while the template of the Style Gallery is not yet applied.

telerik:Label's Context is set to itself and is throwing an error while searching for a not existing 'Title' property within itself.

NullReferenceException when the document contains paragraph elements with no text. 

Fill down (Ctrl-D) is not working in a protected worksheet with an unlocked range of cells.

PrintPreviewControl throws NullReferenceException when it is in the GridView Print example. 

When certain files are imported, an exception with message" is invalid column name" is thrown.

The series is not drawn correctly. The data points are drawn over the same XValue.

Bubble charts are created differently than the way it is in Excel. In excel the data point data is read vertically and in RadSpreadsheet - horizontally.

When the selection contains a certain combination of text and numbers, the charts dialog throws an exception upon opening.

When Bulgarian culture is set and a document with a chart is imported, NullReferenceException is thrown.

Escape key does not close the Insert Chart dialog.

Selecting the whole column and then opening the Charts dialog makes the app hang