From msg512 of issue279, reproduced here for convenience.
Thomas Mittelstaedt writes:
> This happens sometimes, especially with (generated) Makefiles.
> try undo, xemacs tells me that there is no undo information.
> When I reload the buffer, I can't see any visible changes.
> What could that be?
There was a bug that setting the buffer's coding system set the
modified flag, resulting in similar behavior. This would be very
noticable after reverting a buffer. Why this would happen for
generated Makefiles, I'm not sure. Do you revert them often?
I'm referring to was fixed after the release of 21.5.28.
I would appreciate it if you would update to the most recent version
(if you don't have a Mercurial workspace yet, install Mercurial, then
hg clone http://hg.debian.org/hg/xemacs/xemacs
should do the trick). From your description it might be a different
bug, though probably related.
Also, even for something as simple as this it helps to use M-x
report-xemacs-bug, since that will tell us whether the bug should be
fixed in the version you are using. Don't worry about our bandwidth,
and we appreciate you taking the extra effort if it means cutting and
pasting into T-bird or something like that.