The bug

Withers can harm them self and other withers with the explosion created by the impact of a wither skull. This causes a wither to severely damage it self or other withers over time. The wither skull itself cannot harm a wither.

How to reproduce

1. Make a new creative world
2. Spawn a wither. Make sure there are no mobs in the area.
3. Once it has spawned, spawn an iron golem
   → Notice that when the wither fires at it, it will sometimes flash red despite the iron golem not being able to reach that high

A wither at half health or below will be able to hurt itself as well, although more frequently.

Code analysis

Code analysis by marcono1234 cen be found in this comment.