Screwgate Karabiner - Rocky Yellow

I was traversing narrow ledges with fixed ropes in place as handrail/safety line. I had a sling clipped into the line with the Rocky krab clipped into my harness. At a changeover point in the line, I clipped a different sling into the new line and went to unclip the sling in the old line. I noticed the sling was still clipped into the line, but not my harness. I saw the Rocky krab on the ledge next to my foot. On inspection, when I place the krab on its spine, the gate flops down (open). When I place it the opposite way up, the gate does not drop down, but hangs at about 45deg. I can force the gate into the closed position, but on release it springs open. I cannot turn the screwlock at all in either direction.
I have no idea how this happened, but I do know that no significant force was applied and the krab was not weighted at any time. (I was using the krab normally earlier that day.)
I've been using the krab for about a year and externally it seems to be in very good condition. Serial no. 045618.