Jacket type BCD: Shoulder dump valve leaking. Benign dive with hard bottom at 15m, so I used my DS for buoyancy. Unscrewed the valve after the dive, located a loose plastic foil thingy inside, removed it and assembled the valve. Worked fine since that. Still don't know what that plastic foil thingy was, where it came from and whether or not it was supposed to be in there somewhere, though...
Regular buddy's jacket type BCD: Shoulder dump valve leaking. Wall dive with no bottom at rec depths, so I called the dive even though he had redundant buoyancy. Unscrewed the valve, cleaned it for sand and grit, checked the spring, greased the gasket lightly with silicone grease and assembled the valve. Worked fine after that, but we made sure that the next dive was with a hard bottom underneath us.
Never had any issues with my drysuits, except minor leaks between the seals and my skin when I wasn't paying enough attention to the fit while donning. A bit cold-ish to have water trickling in during the dive, but not enough to cause any concern.
Have had quite a few incidents of free-flowing 2nd stages during winter diving, but none of those were underwater. Sounds dramatic, but it isn't. Turn the mouthpiece down and/or stuff your thumb into it, and it'll stop free-flowing at least nine times out of ten.
EDIT:
This questions are for the 500+ dives experts
Why only for them?