It looks like the server doesn't process all the clicks that the client does. This is most visible when you use "raining gold" on a boss. After it's done (with the boss still being alive and well), as much as 75% of the gold gets removed again. This seems to be a bit more than you might expect from a slight timing difference between the client and the server.
I also see this on other occasions where you get a crit which is more than enough to kill the monster, but nothing happens.