500D major image problem

[t3hsauce]

While taking photos of a friend, I encountered a pretty major problem.  Halfway through the shoot I started to see a small underexposed area at the botom of the frame, and thought it might just be due to lighting.  After adjusting lighting (nothing made a difference) I imported the photos into adobe bridge and switched memory cards to see if it made a difference.  The problem has quickly become far worse than it was at the start.  Now, half of the photo is messed up. Has anyone seen this before?  Is something wrong with my sensor?  I can provide a raw file upon request.

[TCampbell]

Are you using flash and exceeding the maximum flash-sync speed?

[ScottyP]

That looks exactly like what suddenly happened to me once.
I was a little embarrassed to find out it was just that the speed lite flash had slid a little backwards in the hot shoe mount. That buggered up the connection just enough to where the flash wasn't talking to the camera about shutter speed or something. I was getting exactly that sort of shutter shadow though.
Is yours possibly not mounted solidly forward in the shoe?