All,
I've been having some repeated problems with my 580EX II flashes, where it changes into TTL exposure mode and essentially loses communications capability with the camera underneath. When this happens, the flash almost always fires far too strong for the picture, and when it's on cameras with menu control, the camera can't seem to communicate with the flash (I get a black screen with a message). Oddly, this seems to happen across at least two of my four flashes, and on three different bodies: a 1D3, a 40D, and a Rebel XTi.
Has anyone seen this? Since the flashes have the new-fangled auto-locking latch on them, I don't think it's a case of the flash being loose. Since I change flashes around a fair amount, I think the contacts are getting enough "action" to stay clean enough. Poor batteries, perhaps?