Slow performance with documents that contain many custom styles. 

Implement pasting of images similar to Excel. 

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.

Investigate if the performance with large files can be improved. Currently a document with a million ros is opened slowly and the memory consumption is high. 

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

Implement functionality to rotate the text in a cell.

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.

In Excel, when you copy a range and then paste it into a larger one, the values are repeated to fill the larger range. This can be included in RadSpreadsheet as well.

Searching with Ctrl-F in a sheet with many rows takes a long time.

When importing a document containing a formula with an invalid reference an exception is thrown: FormulaParseException: 'Unused input [A1] after attempting to parse the formula [#REF!A1]'

According to the Open XML documentation, the possible max value for this attribute is an unsignedInt or 4294967295.

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

The sheet navigation does not work with specific theme and document.

NullReferenceException when the document contains paragraph elements with no text. 

Currently, Custom IconSets are not supported when having Conditional Formatting scenarios.

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