Summoned Wither. His effect did not just damage, it killed players. No other hits or other outside actions causing additional damage. Wither Effect did not stop as it usually does. Killing players with 1 hit from Wither effect. Repeatedly. Players tested and died dozens of times to see exactly how it was occuring. Milk still works to stop effect if done immidiately. Wither effect also seemed to work faster than normal. 

Extreme lag occuring in game during Wither fight. even with only 2 players.

Also, the health bar on the Wither was missing.