Last modified: 2009-06-17 21:37:47 UTC
If somebody edits the header on your talk/discussion/thread/whatever it's supposed to be page you get a "You have new messages." which links to Special:NewMessages, however upon visiting you get: "There are no new messages for you." And the "You have new messages" wont go away.
Fixed up in r51117.
The bug is actually about editing the header, not the talk page.
Resolved in r52051 by not setting newtalk on edits to user talk page headers.