Maybe im just dumb and everyone else already knew this but last night a friend and I were setting up a shot using 2 flashes, a 580ex and a 580ex II . Well while I was getting the on camera flash setup I was being lazy and instead of turning the other flash off I held it against my leg while firing test shots. A few shots later we both smelled something burning...I looked down the flash burnt the dye out of my jeans!!! (and now that dye is on the front of the flash). I had NO clue that could happen. So...heed my warning...
I felt heat but nothing unbearable. (My leg actually had a deep red mark but it wasnt painful) Nothing caught fire but it smelled like it did! They were $70 jeans too : /
On my Rebel XT I used to impress friends by holding a piece of white printer paper nexto the bulb. When it went off there would be a burn mark on the paper. This was the on camera flash!