Do you check
if the "most current" edit is still the same as the one
whos contributions page you are looking at? Otherwise, this might happen:
* User A vandalizes pages, X among them
* Sysop B looks at A's "contributions", with X being a top edit
* Others users have been alerted as well, and just after B loads A's
contributions, user C reverts X manually
* Now, B uses "revert". The top edit is from user C, reverting the
vandalism * So, the revert gets reverted, bringing the vandalism to top
again!
I did a little test, and this kind of behaviour is indeed possible.