Following the steps described further down Witches will target themselves and throw potions straight up into the air basically hitting themselves.
Steps to reproduce:
Preparations: Get witch spawn egg, be in survival.
1. Spawn witch.
2. Hit witch.
3. Change gamemode to creative.
4. Stand right next to the witch so it can hit itself with a damage potion.
5. Wait till witch hits itself with a damage potion
(steps 1-3 need to be done in a quick fashion)
Result: Witch should start throwing potions straight into the air.