Interesting to note that one housing manufacturer seems to be over represented here, especially where there was no blatant user error involved. This agrees with my observations, as well. It always baffles me that people who happily pay $6000+ for a trip will try to save $1500 on a housing which malfunctions or floods, spoiling their trip, and costing them a great deal to replace the ruined camera and lens.
I have also seen a few floods from gross user error when the housing was worked on in the evening, after consuming alcohol. I will never work on my camera after drinking.
I have flooded 2 cameras due to the main housing o-ring popping out of it's groove, one in a test dunk, both due to user error. Now, I remove this o-ring as infrequently as possible during a trip, and have gone 7 years without any more camera floods.
I had a vacuum system installed on my Nauticam housing. It is simple to use, and gave me much confidence during a recent trip to Mexico, where the camera facilities on the liveaboard were decidedly suboptimal. Time will tell if this confidence is misplaced, but I am certain it will detect any large breaches of housing integrity.
I have had continual and ongoing problems with the battery compartment o-ring binding in my Inon 240-Zs while screwing the cap on, resulting in a danger of flooding. Usually this is obvious, but not always. More grease, and new o-rings have not improved this situation.