Excel bug? cell comments become uneditable
 

Sometimes the comment of a cell becomes unusable. I can see the comment as a
tooltip if I hover above the cell, however if I select the cell and press
Shift+F2 to edit it, nothing happens (the tooltip just blinks at every press).

I get this problem periodically for various files, and I haven't been able
to link it to any cause.

I use Excel 2007 but even if I save the file as Excel 2003-compatible and
open it in Excel 2003, the problem remains.

This is most likely a bug in Excel, has anyone encountered it?