I have included two crash reports from the failure. It seems to reproduce reliably. It appears to be new behavior in 1.8.1, as we were not seeing crashes with 1.8. User messy believes the problem occurs when he steps into a nether portal to return to his area in the Overworld.

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 2/11/15 8:38 AM
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
at ayv.f(SourceFile:263)
at are.b(SourceFile:123)
at are.a(SourceFile:59)
at sp.a(SourceFile:514)
at wx.c(SourceFile:1749)
at wx.K(SourceFile:303)
at xo.K(SourceFile:176)
at xq.K(SourceFile:157)
at wx.t_(SourceFile:266)
at xo.t_(SourceFile:1265)
at xq.t_(SourceFile:211)
at afm.t_(SourceFile:32)
at agh.t_(SourceFile:258)
at afn.t_(SourceFile:63)
at aqr.a(SourceFile:1407)
at qu.a(SourceFile:600)
at aqr.g(SourceFile:1385)
at aqr.i(SourceFile:1278)
at qu.i(SourceFile:478)
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
at pp.A(SourceFile:299)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:695)

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

– Head –
Stacktrace:
at ayv.f(SourceFile:263)
at are.b(SourceFile:123)
at are.a(SourceFile:59)
at sp.a(SourceFile:514)
at wx.c(SourceFile:1749)
at wx.K(SourceFile:303)
at xo.K(SourceFile:176)
at xq.K(SourceFile:157)
at wx.t_(SourceFile:266)
at xo.t_(SourceFile:1265)
at xq.t_(SourceFile:211)
at afm.t_(SourceFile:32)
at agh.t_(SourceFile:258)
at afn.t_(SourceFile:63)
at aqr.a(SourceFile:1407)
at qu.a(SourceFile:600)
at aqr.g(SourceFile:1385)

– Entity being ticked –
Details:
Entity Type: PigZombie (afn)
Entity ID: 4348
Entity Name: Zombie Pigman
Entity's Exact location: 523.00, 68.00, -305.00
Entity's Block location: 523.00,68.00,-305.00 - World: (523,68,-305), Chunk: (at 11,4,15 in 32,-20; contains blocks 512,0,-320 to 527,255,-305), Region: (1,-1; contains chunks 32,-32 to 63,-1, blocks 512,0,-512 to 1023,255,-1)
Entity's Momentum: 0.02, -0.08, 0.00
Entity's Rider: ~_ERROR~ NullPointerException: null
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aqr.i(SourceFile:1278)
at qu.i(SourceFile:478)

– Affected level –
Details:
Level name: Someworld
All players: 1 total; [qx['mbessey'/4332, l='Someworld', x=4187.71, y=68.00, z=-2446.18]]
Chunk stats: ServerChunkCache: 750 Drop: 0
Level seed: 2784111450153933165
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: 0.00,64.00,0.00 - World: (0,64,0), Chunk: (at 0,4,0 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: 2006096223 game time, 2024403586 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 118915 (now: false), thunder time: 28516 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
at pp.A(SourceFile:299)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:695)

– System Details –
Details:
Minecraft Version: 1.8.1
Operating System: Mac OS X (x86_64) version 10.9.4
Java Version: 1.6.0_65, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 7679000024 bytes (7323 MB) / 8572502016 bytes (8175 MB) up to 8572502016 bytes (8175 MB)
JVM Flags: 2 total; -Xmx8G -Xms8G
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 20; [qx['mbessey'/4332, l='Someworld', x=4187.71, y=68.00, z=-2446.18]]

---- Minecraft Crash Report ----
// This doesn't make any sense!

Time: 2/10/15 11:24 PM
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
at ayv.f(SourceFile:263)
at are.b(SourceFile:123)
at are.a(SourceFile:59)
at sp.a(SourceFile:514)
at wx.c(SourceFile:1749)
at wx.K(SourceFile:303)
at xo.K(SourceFile:176)
at xq.K(SourceFile:157)
at wx.t_(SourceFile:266)
at xo.t_(SourceFile:1265)
at xq.t_(SourceFile:211)
at afm.t_(SourceFile:32)
at agh.t_(SourceFile:258)
at afn.t_(SourceFile:63)
at aqr.a(SourceFile:1407)
at qu.a(SourceFile:600)
at aqr.g(SourceFile:1385)
at aqr.i(SourceFile:1278)
at qu.i(SourceFile:478)
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
at pp.A(SourceFile:299)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:695)

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

– Head –
Stacktrace:
at ayv.f(SourceFile:263)
at are.b(SourceFile:123)
at are.a(SourceFile:59)
at sp.a(SourceFile:514)
at wx.c(SourceFile:1749)
at wx.K(SourceFile:303)
at xo.K(SourceFile:176)
at xq.K(SourceFile:157)
at wx.t_(SourceFile:266)
at xo.t_(SourceFile:1265)
at xq.t_(SourceFile:211)
at afm.t_(SourceFile:32)
at agh.t_(SourceFile:258)
at afn.t_(SourceFile:63)
at aqr.a(SourceFile:1407)
at qu.a(SourceFile:600)
at aqr.g(SourceFile:1385)

– Entity being ticked –
Details:
Entity Type: PigZombie (afn)
Entity ID: 3119713
Entity Name: Zombie Pigman
Entity's Exact location: 523.00, 68.00, -305.00
Entity's Block location: 523.00,68.00,-305.00 - World: (523,68,-305), Chunk: (at 11,4,15 in 32,-20; contains blocks 512,0,-320 to 527,255,-305), Region: (1,-1; contains chunks 32,-32 to 63,-1, blocks 512,0,-512 to 1023,255,-1)
Entity's Momentum: 0.06, -0.08, -0.01
Entity's Rider: ~_ERROR~ NullPointerException: null
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aqr.i(SourceFile:1278)
at qu.i(SourceFile:478)

– Affected level –
Details:
Level name: Someworld
All players: 2 total; [qx['ItchyBrother'/3091473, l='Someworld', x=-293.48, y=40.37, z=-11141.98], qx['mbessey'/3067986, l='Someworld', x=4188.50, y=68.00, z=-2446.28]]
Chunk stats: ServerChunkCache: 1162 Drop: 0
Level seed: 2784111450153933165
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: 0.00,64.00,0.00 - World: (0,64,0), Chunk: (at 0,4,0 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: 2006076607 game time, 2024383970 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 138531 (now: false), thunder time: 48132 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
at pp.A(SourceFile:299)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:695)

– System Details –
Details:
Minecraft Version: 1.8.1
Operating System: Mac OS X (x86_64) version 10.9.4
Java Version: 1.6.0_65, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 1786491280 bytes (1703 MB) / 8572502016 bytes (8175 MB) up to 8572502016 bytes (8175 MB)
JVM Flags: 2 total; -Xmx8G -Xms8G
IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
Profiler Position: N/A (disabled)
Player Count: 2 / 20; [qx['mbessey'/3067986, l='Someworld', x=4188.50, y=68.00, z=-2446.28], qx['ItchyBrother'/3091473, l='Someworld', x=-293.48, y=40.37, z=-11141.98]]
Is Modded: Unknown (can't tell)
Type: Dedicated Server (map_server.txt)