If you kill a Wither Skeleton, spawned by any means, that has the Health Boost effect on it, no matter how you kill it, it will crash the game.

To Reproduce:

1: Spawn a Wither Skeleton, with the spawn egg, or /summon.

2: Give it Health Boost with the /effect command. (Such as /effect @e[type=skeleton.wither] health_boost 60)

3: Try to kill it.