This had just started happening the other day. One of my 50d's (in great shape other than a dirty sensor), has been freezing when I review pictures.
It gets to a picture (not always the same picture) and totally locks up. I cant press the shutter, or get away from the picture showing up on the LCD screen.
When I turn the camera totally off, the LCD screen is still lit up displaying the picture that the camera froze on. The only way to get rid of this is to remove the battery, and re-install.
Has anyone ever experienced this or have any idea on what the problem is?