It would be nice to be able to undo a revert-buffer operation.
I've just had a situation where I used M-x revert-buffer, and
immediately wanted to see the old version again. (I wanted to briefly
compare the old and new contents, which only differed in several
characters.) I routinely pressed C-_, and got a "no undo data" message
in the echo area. For the shortest instant I was surprised, but I
quickly remembered that M-x revert-buffer clears the undo stack, and
that it has always worked that way.
But I would like to question this design choice. What I wanted to do
was not really unreasonable. The buffer held several kilobytes of data,
and my computer has gigabytes of memory at its disposal, enough room to
hold the buffer contents literally a million times over. If you think
about it, reverting an existing buffer is just another operation on
buffer text, not that different than M-x erase-buffer followed by M-x
insert-file, which would happily remember everything.
If the maintainers agree in principle, I'd like to look into patching
revert-buffer so it records the revert as it would any other change to
the buffer text.