Devil's Lake in Wisconsin is the slipperest rock I have climbed on. I believe it to be a schist. I have seen several cams pull out of seemingly decent placements (the type that would hold at places like the Gunks, or in a decent sandstone).
I was also twice the direct victim of a cam ripping out of a crack due to wetness and it happened both times at the Gunks. First time on a route at Lost City. I fully anticipated it to happen, warned the belayer that it had a high probability of doing so, and had it backed up a few feet lower by a bomber #9 WC Rock. It was still a sobering thing to have happen.
And again it happened while I was attempting The Great Wall of China with Tom Rust in fall of '94. Seconding a pitch down at the far end of the cliff, in somewhat schwaggy territory, and while climbing in a heavy downpour, I had my foot slip as I approached the piece. I literally saw it rip right out of a seemingly secure placement and shoot right at my left eye. I still have a rather nice scar there from the incident. One of the bizarre and comitting things about that climb is that so many times, with run-outs, you can be in a continual and sustained groundfall zone despite being on the 10th+ pitch or so. Just crazy.
Best of all, later that day, bloodied, soaked to the bone, and stoned to the bejeebies, I met my future Mother-in-law for the first time.