Oh, I've heard some interesting stories of gate valve failures.
The most benign is the handle shearing off. It gets worse from there.
I had one where the gate was entirely gone.. Actually got a USCG warning because that particular valve was the head discharge and they could not verify it was closed. I knew it wasn't working properly (it was just above the waterline) but it wasn't until I replaced it that I found the gate was completely gone!