Getting hit by nothing
This happens with every enemy but the worst I experienced was with the beholder. I jumped on the platform and he was chomping another player I was co-opping with. The beholder was so far away from me that I was clearly in. O danger. I died as he chomped the other player. Just fell dead and no enemy anywhere close. I could understand if it were adjacent to me, but I was on the opposite side of the platform and it was facing away from me.