Mobs in my fall trap die on impact but a "ghost" is left on top of the hoppers the hit. Items also seem to be "ghosted". My chests fill up, but I can clearly see the items lying on the hoppers.

This does not happen while I'm looking at the drop zone, but if I stand outside of earshot for a while and then return it is full of "ghost" mobs. These mobs can not be hurt and they don't appear to move. They do however appear in the total entity count if you check F3.

My setup is a 2x2 hole where mobs drop about 24 blocks. The landing area is a 4x4 connection of hoppers leading away all items to be sorted. I have added signs above the hoppers and lava around the edges to kill any armored mobs that might survive.

This is on a server, and while this bug appeared there were other people on besides me. They were however not in the same area.

It seem to be an inconsistent thing, as I managed to duplicate this twice, but while writing this report it has not happened.