I have had this problem also. It occurs infrequently and I have not yet
found any pattern to its occurrence. This is a highly annoying bug, I hope
someone addresses it.
For what it's worth, I found that I can break out the the 'invalid name'
loop by hitting Escape. It allows me to continue using the file without
losing my edits, but it also leaves the objectionable worksheet name intact.
I find that I can then rename the worksheet at a later time.

Signature
Leecu
> I tried to rename my sheet and it popped up a .xls in front of the name for
> the tab. I deleted the .xls (Mind you this is just the name on the tab for
> one sheet in
> a series). It now won't accept anyname and keeps telling me to I should use
> blanks or certain characters. I have tried single letters, single numbers.
> Nothing works and it will not let me out of the sheet.
Leecu - 23 Aug 2006 21:29 GMT
Check this thread:
"Prefix .xls] after renaming a sheet?" in Excel General Questions
The problem may be bracket characters in the filename.

Signature
Leecu
> I have had this problem also. It occurs infrequently and I have not yet
> found any pattern to its occurrence. This is a highly annoying bug, I hope
[quoted text clipped - 12 lines]
> > blanks or certain characters. I have tried single letters, single numbers.
> > Nothing works and it will not let me out of the sheet.
Bill Krayer - 23 Aug 2006 23:57 GMT
I found a fix. It happens when I open a spreadsheet that has been emailed to
me. If I save the spreadsheet to my harddrive, before I play with it, then I
have no problem with the Tab name change.
Thanks for the response!
Bill
> Check this thread:
>
[quoted text clipped - 18 lines]
> > > blanks or certain characters. I have tried single letters, single numbers.
> > > Nothing works and it will not let me out of the sheet.