You never heard about of all those canon 5Ds that died suddenly on an Antarctic cruise did you? They were only in very very light drizzle and had a very high failure rate among the large group that was using canons. The Nikon's in the group didn't have a single failure.
In a summary session on the last day at the Peninsula I asked everyone to report on any equipment failures. Here's the tally.
The top LCD on a 5D MKII spontaneously cracked; Another 5D MKII had a jambed on lens caused by a loose screw, a 1Ds MKIII reported intermittent problems; a 1D MKIII kept reporting Error 99; one Hasselblad reported electronic lens connection problems; two Canon G9's failed (no G10s had any reported problems), and a Nikon 80-400mm lens came apart. No Nikon bodies (mostly D700s) failed in any way.
The largest group of failures through were among the Canon 5D MKIIs. Of the 26 samples of this camera onboard, one quarter (six) failed at one time or another, and while three recovered, the other three never did. In all cases it appeared to be water or humidity damage. Of particular concern were two cameras which stopped working while completely protected within Kata rain covers during a light rain ashore. They came back to life the following day though and were mostly fine for the rest of the trip, but one died permenently just before the end of our voyage.
Several people noted that when returning to the ship after working in light rain 5D MKIIs with vertical battery grips tended to collect water in between the grip and the base – something that may have been the cause of some of the failures.
So YES there is such a thing as catastrophic failure due to moisture.