I use the Classic skin on enwiki, and when I get the "You have new messages"
alert at the top of the screen, the diff link next to it doesn't give a diff.
Instead, it gives another link to your talk page. The last time this happened I
had just logged in, but this has happened other times all across en:Wikipedia. I
know that at least one other user has had this problem, [[en:User:Geni]] (see
[[en:MediaWiki talk:Youhavenewmessages]]. I'm assuming that instead of another
link to your talk page, one's supposed to be getting a diff of the last edit.
I reproduced the bug by editing my talk page under another account
and then logging back in. I then repeated the test under the monobook skin, and
the diff link worked fine.