Setting a spider den aflame and trying to pick up the spiders after they fall asleep will result in a server crash.
The spiders you pick up don't show up in the inventory but webber says that his inventory is already full.
After this you are unable to pick up items and after 10 seconds or so the server will crash.
Updated, If you use a pan fluit to make the spiders sleep they also crash the game when picked up. To crash the server you need to leave it then everyone else crashes.
Be in an online server, I joined a server of a friend of mine.
Have a spider den (The one I used was tier 3)
Have a torch and set the den aflame (Before the rollback of our server lightning struck the den so that also is viable)
Using a panfluit to make the spiders sleep also makes them break the game.
Try to pick up the spiders after they have fallen asleep. (It has to be day)
Then leave the server, everyone else crashes after that.