This one involves an Act 3 spoiler.
Killiing Gortash before destroying the Steel Watch factory! I gleefully shoved him off of Wyrm's Rock, but in that moment the Netherbrain destroyed the factory itself and took control of the Steel Watch, making them permanently aggro throughout the city and force me into near-constant battle any time I wanted to just walk anywhere until I cleared them out!
And non-spoiler, I tried building a Fighter/Paladin multiclass and it was just not good. Trying to build for both means sacrificing high level abilities, which resulted in a character weaker than pure Fighter or pure Paladin.
