I've got the same issue as stepurhan - noticed it the other day. Just had another look and it's still the case (I emptied my cache yesteday too).
Example:
I last posted on
this thread on Feb 6, but the
Chit Chat Index shows me as last poster on Feb 12.
There are some others that show me as the last poster when I'm definitely not but I can't spot any right now.
Not a big issue though. Just a small tear in the fabric of spacetime I suspect.