I had a tripped water sensor in my 3GS. Recently, the phone would vibrate/switch between silent/non-silent mode at the slightest touch. I could squeeze it and make it vibrate. I took in into the store expecting them to send me home because I had a tripped moisture sensor (which I think was unrelated). The Genius noticed the sensor, and said that she needed to check the phone out further. She took it in the back for a minute or so, and came out with a brand new replacement. I'm really pleased with on this one, for not using the tripped sensor to get out from my warranty.