Put the summary of the bug you're having here
What I expected to happen was...:
Get on my world with cutsom spawners
What actually happened was...:
The game crashed when my spawners loaded
Steps to Reproduce:*
1. make a spawner with custom NBT data in McEdit
2. Get on the world
3. The game will crash when the spawner is loaded
This crash only seems to happen when the mob has custom potion effects. I have some skeletons with custom armor and enchanted bows which don't cause the crash, but my Speed IV zombies do.