I had a very strange failure on my 5D today during a shoot.
Part way through the shoot the data display in the viewfinder went blank (as though the camera had powered off). However the LCD display on the top of the camera was still active and working. Autofocus stopped working and the shutter release button didn’t work.
I had my Speedlight 550EX flash gun on the camera to act as fill light and also to trigger a couple of “umbrella flash” units.
When I removed the flash gun from the camera the 5D worked normally. Put the flash gun back on the camera and again everything except the LCD display stopped working. At this point I decided to switch over to using a sync cable to trigger the brolly flashes and abandon the Speedlight. However when the cable was connected to the PC socket, again the display, AF, and shutter release failed.
Because I didn’t want to keep my paying customer waiting any longer I switched to my backup camera (10D) to complete the shoot (using the same lens and the speedlight). At end of the shoot I went to investigate the problem on the 5D and could not reproduce it.
I hate intermitent problems.