Bent seat tube
 

As I was locking up a few hours ago I noticed that my seat tube is bowed out about 1/8" towards the rear wheel and that there are two dents (one of each side) on the rear of the seat tube right below where it meets the TT. I can't see any cracked/chipped paint at the welds where it would be more likely to give under stress, nor any marks on the paint where a thief might have been prying on it. What the **** did I do? The frame still seems straight side to side. Steel frame.

How could I possibly damage that tube?

That sucks. Have you been running a too short seatpost?

No sir. There's at least 10cm in the tube.

have you been putting on weight recently?

Maybe your bike's been cheating on you with a fat chick.

How close is this to your seat stays, and how do they attach to the seat cluster?
One of my bikes with a fastback style cluster had something which sounds similar, turns out that the seat stays had punched holes through the seat tube, and the metal and crumpled beneth.
Got any photos, take out the seatpost and have a look from the inside.

--Josh