Management Studio sometimes displays this error when you try to copy text from SQL Source Control using the Ctrl + C keyboard shortcut.

Workaround

Copy the text using the right-click menu.

Solution

To stop this happening:

1. Close all query windows.
   Alternatively, you can restart SSMS after following these steps. 
2. Go to Tools -> Options.
3. Under the Environment section, select Documents.
4. Deselect the Show Miscellaneous files in Solution Explorer checkbox:
   
5. Click OK.

Copy and paste should now work normally when there are unsaved query windows open.