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A search here turned up nothing, so I'm posting this just as an FYI and "public service announcment".
I found a small problem that will probably happen to other 5d owners in the near future. We all know that our cameras use internal batteries, but I've never had one die on a digital body (I usually don't keep them long enough). The battery maintains "some" of our settings , but......
When I tried to use my 5d yesterday, I found all the settings were changed. I thought perhaps someone might have changed things on me when they grabbed the camera, but when I tried to change the settings back nothing would stick. The camera quality mode went to jpeg vs. RAW, the histogram changed to brightness vs. RGB, the focus points changed (I leave it on center only) and the custom functions were all wiped out.
Long story short, as I suspected, it was just the internal, or backup, battery ( CR2016, CR-2016, or 2016) that needed to be replaced. If you lose your settings on a 5d, or other body over 1-2 years old, start by replacing the battery. It's not serious and will cost $2.00 to fix.
I hope this might help someone when they see strange camera behavior.