I'm building an Etho style gold farm in the nether. Everything went smooth up until I placed down 2 iron golems in the cage. Immediately after the second golem came to life, the game crashed and my world was corrupted.

I was able to recover the world by copying the files level.dat and level.dat_old from an older save (these files were now missing in my world save directory).

After booting up my recovered world I headed back to the gold farm. The game crashed again almost instantly. I think I saw one of the iron golems hit a zombie pigman just before the crash.

Crash report
---- Minecraft Crash Report ----
// Quite honestly, I wouldn't worry myself about that.

Time: 01.11.15 22:17
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
at ry.a(SourceFile:445)
at rx.a(SourceFile:833)
at zf.a(SourceFile:634)
at rx.a(SourceFile:766)
at yw.a(SourceFile:61)
at zf.a(SourceFile:235)
at yx.a(SourceFile:148)
at we.B(SourceFile:167)
at to.e(SourceFile:121)
at th.a(SourceFile:129)
at ry.cd(SourceFile:620)
at rx.n(SourceFile:1680)
at ry.n(SourceFile:473)
at we.n(SourceFile:120)
at rx.m(SourceFile:1542)
at ry.m(SourceFile:265)
at aho.a(SourceFile:1481)
at lq.a(SourceFile:719)
at aho.g(SourceFile:1459)
at aho.k(SourceFile:1349)
at lq.k(SourceFile:552)
at net.minecraft.server.MinecraftServer.D(SourceFile:618)
at net.minecraft.server.MinecraftServer.C(SourceFile:546)
at bya.C(SourceFile:154)
at net.minecraft.server.MinecraftServer.run(SourceFile:450)
at java.lang.Thread.run(Thread.java:745)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Stacktrace:
at ry.a(SourceFile:445)
at rx.a(SourceFile:833)
at zf.a(SourceFile:634)
at rx.a(SourceFile:766)
at yw.a(SourceFile:61)
at zf.a(SourceFile:235)
at yx.a(SourceFile:148)
at we.B(SourceFile:167)
at to.e(SourceFile:121)
at th.a(SourceFile:129)
at ry.cd(SourceFile:620)
at rx.n(SourceFile:1680)
at ry.n(SourceFile:473)
at we.n(SourceFile:120)
at rx.m(SourceFile:1542)
at ry.m(SourceFile:265)
at aho.a(SourceFile:1481)
at lq.a(SourceFile:719)
at aho.g(SourceFile:1459)

– Entity being ticked –
Details:
Entity Type: VillagerGolem (we)
Entity ID: 124541
Entity Name: Iron Golem
Entity's Exact location: -6,97, 202,00, 401,14
Entity's Block location: World: (-7,202,401), Chunk: (at 9,12,1 in -1,25; contains blocks -16,0,400 to -1,255,415), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Entity's Momentum: 0,01, -0,08, 0,00
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aho.k(SourceFile:1349)
at lq.k(SourceFile:552)

– Affected level –
Details:
Level name: <worldname>
All players: 1 total; [ls['<playername>'/475, l='<worldname>', x=-35,37, y=200,00, z=394,77]]
Chunk stats: ServerChunkCache: 2242 Drop: 0
Level seed: -8242230892120991772
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (0,0,0), Chunk: (at 0,0,0 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 0 game time, 0 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: ~_ERROR~ NullPointerException: null
Stacktrace:
at net.minecraft.server.MinecraftServer.D(SourceFile:618)
at net.minecraft.server.MinecraftServer.C(SourceFile:546)
at bya.C(SourceFile:154)
at net.minecraft.server.MinecraftServer.run(SourceFile:450)
at java.lang.Thread.run(Thread.java:745)

– System Details –
Details:
Minecraft Version: 15w44b
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 481000400 bytes (458 MB) / 1138573312 bytes (1085 MB) up to 4281597952 bytes (4083 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx4G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [ls['<playername>'/475, l='<worldname>', x=-35,37, y=200,00, z=394,77]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.