Minecraft 15w36a, Minecraft 15w36c
Java 8u25, Windows 10
Basically what the Summary says.
Steps to reproduce:
1. Summon any peaceful mod (other then yourself) (or use a already existing one)
2. Summon Iron Golem (or use a already existing one)
3. Get the snow golem to the sight of the iron golem
Iron Golem attacks the peaceful mob for no reason at all.