Here's the crash report:

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

Time: 10/21/14 12:57 PM
Description: Exception ticking world

java.lang.IllegalArgumentException: n must be positive
at java.util.Random.nextInt(Random.java:250)
at aia.a(SourceFile:122)
at aia.a(SourceFile:27)
at ahu.a(SourceFile:364)
at aip.a(SourceFile:74)
at aqz.a(SourceFile:432)
at ms.a(SourceFile:166)
at apx.a(SourceFile:772)
at ms.c(SourceFile:101)
at ms.d(SourceFile:114)
at ahb.e(SourceFile:310)
at mt.g(SourceFile:234)
at mt.b(SourceFile:137)
at net.minecraft.server.MinecraftServer.v(SourceFile:514)
at net.minecraft.server.MinecraftServer.u(SourceFile:453)
at bsx.u(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:369)
at lj.run(SourceFile:628)

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

– Head –
Stacktrace:
at java.util.Random.nextInt(Random.java:250)
at aia.a(SourceFile:122)
at aia.a(SourceFile:27)
at ahu.a(SourceFile:364)
at aip.a(SourceFile:74)
at aqz.a(SourceFile:432)
at ms.a(SourceFile:166)
at apx.a(SourceFile:772)
at ms.c(SourceFile:101)
at ms.d(SourceFile:114)
at ahb.e(SourceFile:310)
at mt.g(SourceFile:234)
at mt.b(SourceFile:137)

– Affected level –
Details:
Level name: The Norton Bro Islands
All players: 2 total; [mw['spyguy8404'/55, l='The Norton Bro Islands', x=-624.08, y=117.62, z=-111.01], mw['daffyducky8404'/9152, l='The Norton Bro Islands', x=-585.30, y=244.00, z=-514.24]]
Chunk stats: ServerChunkCache: 1337 Drop: 0
Level seed: 202961
Level generator: ID 02 - largeBiomes, ver 0. Features enabled: false
Level generator options:
Level spawn location: World: (-12,172,-296), Chunk: (at 4,10,8 in -1,-19; contains blocks -16,0,-304 to -1,255,-289), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Level time: 23619 game time, 2891 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 27464 (now: false), thunder time: 64684 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.v(SourceFile:514)
at net.minecraft.server.MinecraftServer.u(SourceFile:453)
at bsx.u(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:369)
at lj.run(SourceFile:628)

– System Details –
Details:
Minecraft Version: 1.7.10
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: 99927784 bytes (95 MB) / 294584320 bytes (280 MB) up to 3216900096 bytes (3067 MB)
JVM Flags: 1 total; -Xmx3G
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 103
Profiler Position: N/A (disabled)
Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Player Count: 2 / 8; [mw['spyguy8404'/55, l='The Norton Bro Islands', x=-624.08, y=117.62, z=-111.01], mw['daffyducky8404'/9152, l='The Norton Bro Islands', x=-585.30, y=244.00, z=-514.24]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.