Put the summary of the bug you're having here
What I expected to happen was...:
The world would load and work normally
What actually happened was...:
The moment i loaded the world it crashed
Steps to Reproduce:
1. Spawn a mob
2. Put a potion effect on it
3. turn it into a spawner
Environment
OS: Windows 8 (ver 6.2, arch x86)
Java: 1.7.0_21 (by Oracle Corporation)
Launcher: 0.8.3 (Dev) (bootstrap 2)
Minecraft: 13w23a (updated Fri Jun 07 09:04:20 PDT 2013)
Based on a crash report provided in this issue, I believe with high certainty that this report may be a duplicate of MC-17399. You can view all information known about this specific crash at MCX-659.