I have one specific Excel file which generates the following very buggy behavior:

Anytime I close the file in Excel, Excel itself remains open with a grey screen (no file open). When I try to close the whole program, it asks again if I want to save the file which has already been saved and closed. If I choose "Save file", the next time I try to open that file I get the error "Run-time error 13: Type mismatch".

Fortunately, I had a backup the first time this happened and was able to restore the file. I have tested it repeatedly and it is only with this specific file and filename; if I store the same file under a different name, it closes properly and Excel does not ask to save it a second time.

I have tried restarting the computer, repairing Office, etc. Nothing helps.

The solution for me is easy; I have renamed the file and all is well. But you may want to fix the underlying bug!