I died twice when trying to climb up a ladder, for no apparent reason. First time I was climbing (didn't stop) and about midway the screen went black "You are dead". Second time I climbed to almost the top, stopped and looked around using "Alt"+mouse movement. After a second the screen went black "You are dead".

Please note that I did not see myself falling before I died, screen went blank while I was on the ladder, as if I was shot. However, I had a friend around in both cases and nobody else on the server. Friend said he saw my body fall to the ground about a second after I died.