-snip-
Edit:
Just clicked some more threads, and I know what is happening!
It is caused by "SatoshiGames" AD, it is too wide, so the scroll bar shows up.

Can you fix this?
You are saying that this only happens on pages on which the SatoshiGames AD is being shown (as in, it is fine with all other ADs and on pages where their AD is not shown)? I'm not sure whether this breaks the auction rules for ADs because your width is lower than the one specified:
Ads must appear 3 or fewer lines tall in my browser (Firefox, 900px wide)
In any case, I've sent a message to theymos.