There was a problem with a 5d (Mk I) body. The 2. contact of the exposure button was not working.
After disassembling there was nothing to see, but verified with a multimeter there was no contact by pressing it to the second position. Stage 1. was OK.
The contacts are made of spring-steel (or highly elastic steel in any case), the end is gold plated. there was no sign of corrosion.
I soldered the other end to the flex-cable. Checked with the multimeter – it OK now.
I assembled the camera, a new check – it was working.
It’s very annoying, You can loose a lot of picture because a simple could soldering point.
In the last 3 years I’ve seen 2 pieces of 5d MkI and 1 piece of 1D Mk II with this problem.