After a Zoglin is attacked by a horde of silverfish, some will not move after the Zoglin has been killed. If a player moves into the aggro range of the then-immobile silverfish, their AI will resume to normal. Should be easy enough to recreate.
Code analysis by PR0CESS in this comment
reinforcement Baby zombie spawned as in if NoAi when spawned in mushroom fields if player dies before pathfinding occurs
Vex continue to attack their target after it has been killed