Sometimes, in minecraft 15w45a, random crashes occur. it's happened too me on my server and in singleplayer.
here's a crash report from a gold p farm world:

---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 12-11-15 21:02
Description: Ticking entity

java.lang.ClassCastException: ahw cannot be cast to ahn
at aqo.a(SourceFile:148)
at aqv$a.c(SourceFile:373)
at aym.a(SourceFile:79)
at ayk.a(SourceFile:82)
at ayk.a(SourceFile:40)
at ayk.a(SourceFile:30)
at vb.a(SourceFile:89)
at va.a(SourceFile:67)
at va.a(SourceFile:76)
at to.a(SourceFile:50)
at th.a(SourceFile:94)
at ry.cd(SourceFile:631)
at rx.n(SourceFile:1688)
at ry.n(SourceFile:484)
at yv.n(SourceFile:35)
at zf.n(SourceFile:243)
at rx.m(SourceFile:1550)
at ry.m(SourceFile:275)
at yv.m(SourceFile:40)
at zf.m(SourceFile:299)
at yw.m(SourceFile:68)
at ahn.a(SourceFile:1481)
at lq.a(SourceFile:719)
at ahn.g(SourceFile:1459)
at ahn.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 bxx.C(SourceFile:154)
at net.minecraft.server.MinecraftServer.run(SourceFile:450)
at java.lang.Thread.run(Unknown Source)

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

– Head –
Stacktrace:
at aqo.a(SourceFile:148)
at aqv$a.c(SourceFile:373)
at aym.a(SourceFile:79)
at ayk.a(SourceFile:82)
at ayk.a(SourceFile:40)
at ayk.a(SourceFile:30)
at vb.a(SourceFile:89)
at va.a(SourceFile:67)
at va.a(SourceFile:76)
at to.a(SourceFile:50)
at th.a(SourceFile:94)
at ry.cd(SourceFile:631)
at rx.n(SourceFile:1688)
at ry.n(SourceFile:484)
at yv.n(SourceFile:35)
at zf.n(SourceFile:243)
at rx.m(SourceFile:1550)
at ry.m(SourceFile:275)
at yv.m(SourceFile:40)
at zf.m(SourceFile:299)
at yw.m(SourceFile:68)
at ahn.a(SourceFile:1481)
at lq.a(SourceFile:719)
at ahn.g(SourceFile:1459)

– Entity being ticked –
Details:
Entity Type: PigZombie (yw)
Entity ID: 164
Entity Name: Zombie Pigman
Entity's Exact location: 27,70, 239,00, 101,30
Entity's Block location: World: (27,239,101), Chunk: (at 11,14,5 in 1,6; contains blocks 16,0,96 to 31,255,111), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Entity's Momentum: 0,00, -0,08, 0,00
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at ahn.k(SourceFile:1349)
at lq.k(SourceFile:552)

– Affected level –
Details:
Level name: GoldXP
All players: 1 total; [ls['xandertje10'/112, l='GoldXP', x=26,70, y=245,00, z=100,86]]
Chunk stats: ServerChunkCache: 1126 Drop: 0
Level seed: -2462177341800582946
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 bxx.C(SourceFile:154)
at net.minecraft.server.MinecraftServer.run(SourceFile:450)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 15w45a
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.8.0_65, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 363708744 bytes (346 MB) / 685056000 bytes (653 MB) up to 3207856128 bytes (3059 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx3G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 1, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [ls['xandertje10'/112, l='GoldXP', x=26,70, y=245,00, z=100,86]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.