I've been twice bitten by wired smoke detectors.
The first time, I figured they were low voltage and paid them no respect. Found out they are 120v AC.
Second time, I was standing on the top platform of a 6' fiberglass ladder and figured I was isolated from ground, and operating with a single hand, so no danger. Zapped me through a single finger while making the connection and nearly jumped off the ladder, but just barely stayed on. *slow learner with electricity