I was hosting a LAN world and was searching for an end monument, when an enderman decided to attack me. He teleported, but he left a trail of particles (as if he was walking in one direction and emitting particles the entire time). Soon afterward, minecraft started to lag to the point where it became unplayable. When I opened the pause menu, it still lagged and I got about a frame per every few seconds. When I looked in the launcher log, it was COMPLETELY filled with the following:
[Server thread/WARN]: Tried to add an entity with duplicate UUID <uuid>
When I restarted minecraft and loaded the world again, all of the lag had completely disappeared, and I was not able to replicate the bug.