When I first fought Skeletron Prime, its movements were so erratic that it missed abilities. Yet, it suddenly spawned the Twins once Skeletron reached phase three, which became invincible due to the absurd health regeneration.

Here is the list of bosses considered broken:
The Twins - Triggers Infernum Animation even though Infernum is not active.
Skeletron Prime - Unusual and unpredictable patterns. It regenerates more health as it reaches the second and third phases, which leads to becoming invincible and summoning the Twins. Lastly, it enrages too.