Is the problem repeatable? Might make a copy of that file (just in case it
is repeatable) and then try the edit of the comment again on the original to
see if it misbehaves that way again. Also, if it does apparently delete data
on the sheet, instead of using UNDO immediately, choose a few of the cells
and see if their contents actually got deleted or if the video display simply
isn't showing them.
This is a new one as far as my personal knowledge goes - haven't seen this
issue mentioned before (but doesn't mean much, as I don't see every posting).
This probably means it either isn't happening to many other people and if it
is for-real happening to you, it probably takes a unique setup of the
contents/format whatever of the workbook/worksheet to cause the as yet
undocumented "clear part but not all of my information" feature of Excel 2007
to show itself.