I have no idea as it was a random crash. This is the report in the crash report section of the launcher:

---- Minecraft Crash Report ----
// Don't be sad. I'll do better next time, I promise!

Time: 7/3/13 10:19 PM
Description: Exception in world tick

java.lang.IllegalStateException: Player is either on another team or not on any team. Cannot remove from team 'hbr0'.
	at atc.b(SourceFile:202)
	at bcn.a(SourceFile:1173)
	at ft.a(SourceFile:99)
	at cn.b(SourceFile:350)
	at bcn.d(SourceFile:95)
	at bcu.b(SourceFile:60)
	at atn.k(SourceFile:1380)
	at atn.S(SourceFile:662)
	at atn.d(SourceFile:618)
	at net.minecraft.client.main.Main.main(SourceFile:101)


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

-- Head --
Stacktrace:
	at atc.b(SourceFile:202)
	at bcn.a(SourceFile:1173)
	at ft.a(SourceFile:99)
	at cn.b(SourceFile:350)
	at bcn.d(SourceFile:95)

-- Affected level --
Details:
	Level name: MpServer
	All players: 4 total; [bcx['Funnygatt'/26135, l='MpServer', x=-779.37, y=31.62, z=-1183.62], ben['N_C_'/405374, l='MpServer', x=-778.56, y=30.00, z=-1183.50], ben['Bob_Bobson99'/399338, l='MpServer', x=-768.53, y=17.00, z=-1183.97], ben['Beckforce2'/396464, l='MpServer', x=-757.06, y=27.00, z=-1180.97]]
	Chunk stats: MultiplayerChunkCache: 441
	Level seed: 0
	Level generator: ID 01 - flat, ver 0. Features enabled: false
	Level generator options: 
	Level spawn location: World: (-779,31,-1184), Chunk: (at 5,1,0 in -49,-74; contains blocks -784,0,-1184 to -769,255,-1169), Region: (-2,-3; contains chunks -64,-96 to -33,-65, blocks -1024,0,-1536 to -513,255,-1025)
	Level time: 108622656 game time, 109868617 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: Game mode: adventure (ID 2). Hardcore: false. Cheats: false
	Forced entities: 33 total; [oa['entity.ItemFrame.name'/0, l='MpServer', x=-803.50, y=20.50, z=-1178.94], oa['entity.ItemFrame.name'/1, l='MpServer', x=-793.94, y=18.50, z=-1184.50], oa['entity.ItemFrame.name'/2, l='MpServer', x=-792.50, y=18.50, z=-1184.94], oa['entity.ItemFrame.name'/3, l='MpServer', x=-793.50, y=18.50, z=-1184.94], ben['N_C_'/405374, l='MpServer', x=-778.56, y=30.00, z=-1183.50], ben['Beckforce2'/396464, l='MpServer', x=-757.06, y=27.00, z=-1180.97], ben['Bob_Bobson99'/399338, l='MpServer', x=-768.53, y=17.00, z=-1183.97], oa['entity.ItemFrame.name'/4, l='MpServer', x=-793.94, y=18.50, z=-1183.50], oa['entity.ItemFrame.name'/5, l='MpServer', x=-793.94, y=18.50, z=-1182.50], oa['entity.ItemFrame.name'/6, l='MpServer', x=-792.50, y=18.50, z=-1182.06], oa['entity.ItemFrame.name'/7, l='MpServer', x=-793.50, y=18.50, z=-1182.06], ben['Beckforce2'/396464, l='MpServer', x=-757.06, y=27.00, z=-1180.97], oa['entity.ItemFrame.name'/8, l='MpServer', x=-790.94, y=20.50, z=-1183.50], oa['entity.ItemFrame.name'/9, l='MpServer', x=-777.50, y=20.50, z=-1172.06], oa['entity.ItemFrame.name'/10, l='MpServer', x=-779.50, y=20.50, z=-1172.06], oa['entity.ItemFrame.name'/11, l='MpServer', x=-763.06, y=18.50, z=-1184.50], oa['entity.ItemFrame.name'/12, l='MpServer', x=-763.06, y=19.50, z=-1184.50], oa['entity.ItemFrame.name'/13, l='MpServer', x=-766.06, y=17.50, z=-1184.50], oa['entity.ItemFrame.name'/14, l='MpServer', x=-767.06, y=20.50, z=-1185.50], oa['entity.ItemFrame.name'/15, l='MpServer', x=-763.06, y=19.50, z=-1183.50], oa['entity.ItemFrame.name'/17, l='MpServer', x=-763.06, y=18.50, z=-1183.50], oa['entity.ItemFrame.name'/16, l='MpServer', x=-763.06, y=19.50, z=-1182.50], oa['entity.ItemFrame.name'/19, l='MpServer', x=-766.06, y=19.50, z=-1183.50], oa['entity.ItemFrame.name'/18, l='MpServer', x=-763.06, y=18.50, z=-1182.50], oa['entity.ItemFrame.name'/21, l='MpServer', x=-766.06, y=17.50, z=-1182.50], oa['entity.ItemFrame.name'/20, l='MpServer', x=-766.06, y=17.50, z=-1183.50], oa['entity.ItemFrame.name'/23, l='MpServer', x=-767.06, y=20.50, z=-1181.50], oa['entity.ItemFrame.name'/22, l='MpServer', x=-767.06, y=20.50, z=-1183.50], bcx['Funnygatt'/26135, l='MpServer', x=-779.37, y=31.62, z=-1183.62], ben['Bob_Bobson99'/399338, l='MpServer', x=-768.53, y=17.00, z=-1183.97], ben['burningbow32569'/407944, l='MpServer', x=-795.94, y=27.00, z=-1192.06], ben['N_C_'/405374, l='MpServer', x=-778.56, y=30.00, z=-1183.50], ben['awesome_ender'/407929, l='MpServer', x=-797.54, y=27.00, z=-1190.88]]
	Retry entities: 0 total; []
Stacktrace:
	at bcu.a(SourceFile:284)
	at atn.k(SourceFile:1387)
	at atn.S(SourceFile:662)
	at atn.d(SourceFile:618)
	at net.minecraft.client.main.Main.main(SourceFile:101)

-- Affected level --
Details:
	Level name: MpServer
	All players: 4 total; [bcx['Funnygatt'/26135, l='MpServer', x=-779.37, y=31.62, z=-1183.62], ben['N_C_'/405374, l='MpServer', x=-778.56, y=30.00, z=-1183.50], ben['Bob_Bobson99'/399338, l='MpServer', x=-768.53, y=17.00, z=-1183.97], ben['Beckforce2'/396464, l='MpServer', x=-757.06, y=27.00, z=-1180.97]]
	Chunk stats: MultiplayerChunkCache: 441
	Level seed: 0
	Level generator: ID 01 - flat, ver 0. Features enabled: false
	Level generator options: 
	Level spawn location: World: (-779,31,-1184), Chunk: (at 5,1,0 in -49,-74; contains blocks -784,0,-1184 to -769,255,-1169), Region: (-2,-3; contains chunks -64,-96 to -33,-65, blocks -1024,0,-1536 to -513,255,-1025)
	Level time: 108622656 game time, 109868617 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: Game mode: adventure (ID 2). Hardcore: false. Cheats: false
	Forced entities: 33 total; [oa['entity.ItemFrame.name'/0, l='MpServer', x=-803.50, y=20.50, z=-1178.94], oa['entity.ItemFrame.name'/1, l='MpServer', x=-793.94, y=18.50, z=-1184.50], oa['entity.ItemFrame.name'/2, l='MpServer', x=-792.50, y=18.50, z=-1184.94], oa['entity.ItemFrame.name'/3, l='MpServer', x=-793.50, y=18.50, z=-1184.94], ben['N_C_'/405374, l='MpServer', x=-778.56, y=30.00, z=-1183.50], ben['Beckforce2'/396464, l='MpServer', x=-757.06, y=27.00, z=-1180.97], ben['Bob_Bobson99'/399338, l='MpServer', x=-768.53, y=17.00, z=-1183.97], oa['entity.ItemFrame.name'/4, l='MpServer', x=-793.94, y=18.50, z=-1183.50], oa['entity.ItemFrame.name'/5, l='MpServer', x=-793.94, y=18.50, z=-1182.50], oa['entity.ItemFrame.name'/6, l='MpServer', x=-792.50, y=18.50, z=-1182.06], oa['entity.ItemFrame.name'/7, l='MpServer', x=-793.50, y=18.50, z=-1182.06], ben['Beckforce2'/396464, l='MpServer', x=-757.06, y=27.00, z=-1180.97], oa['entity.ItemFrame.name'/8, l='MpServer', x=-790.94, y=20.50, z=-1183.50], oa['entity.ItemFrame.name'/9, l='MpServer', x=-777.50, y=20.50, z=-1172.06], oa['entity.ItemFrame.name'/10, l='MpServer', x=-779.50, y=20.50, z=-1172.06], oa['entity.ItemFrame.name'/11, l='MpServer', x=-763.06, y=18.50, z=-1184.50], oa['entity.ItemFrame.name'/12, l='MpServer', x=-763.06, y=19.50, z=-1184.50], oa['entity.ItemFrame.name'/13, l='MpServer', x=-766.06, y=17.50, z=-1184.50], oa['entity.ItemFrame.name'/14, l='MpServer', x=-767.06, y=20.50, z=-1185.50], oa['entity.ItemFrame.name'/15, l='MpServer', x=-763.06, y=19.50, z=-1183.50], oa['entity.ItemFrame.name'/17, l='MpServer', x=-763.06, y=18.50, z=-1183.50], oa['entity.ItemFrame.name'/16, l='MpServer', x=-763.06, y=19.50, z=-1182.50], oa['entity.ItemFrame.name'/19, l='MpServer', x=-766.06, y=19.50, z=-1183.50], oa['entity.ItemFrame.name'/18, l='MpServer', x=-763.06, y=18.50, z=-1182.50], oa['entity.ItemFrame.name'/21, l='MpServer', x=-766.06, y=17.50, z=-1182.50], oa['entity.ItemFrame.name'/20, l='MpServer', x=-766.06, y=17.50, z=-1183.50], oa['entity.ItemFrame.name'/23, l='MpServer', x=-767.06, y=20.50, z=-1181.50], oa['entity.ItemFrame.name'/22, l='MpServer', x=-767.06, y=20.50, z=-1183.50], bcx['Funnygatt'/26135, l='MpServer', x=-779.37, y=31.62, z=-1183.62], ben['Bob_Bobson99'/399338, l='MpServer', x=-768.53, y=17.00, z=-1183.97], ben['burningbow32569'/407944, l='MpServer', x=-795.94, y=27.00, z=-1192.06], ben['N_C_'/405374, l='MpServer', x=-778.56, y=30.00, z=-1183.50], ben['awesome_ender'/407929, l='MpServer', x=-797.54, y=27.00, z=-1190.88]]
	Retry entities: 0 total; []

-- System Details --
Details:
	Minecraft Version: 1.6.1
	Operating System: Mac OS X (x86_64) version 10.8.4
	Java Version: 1.6.0_51, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 48530320 bytes (46 MB) / 122593280 bytes (116 MB) up to 1060372480 bytes (1011 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 26617 (1490552 bytes; 1 MB) allocated, 25 (1400 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
	Launched Version: 1.6.1
	LWJGL: 2.9.0
	OpenGL: NVIDIA GeForce GTX 675MX OpenGL Engine GL version 2.1 NVIDIA-8.12.47 310.40.00.05f01, NVIDIA Corporation
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Pack: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 852 (47712 bytes; 0 MB) allocated, 101 (5656 bytes; 0 MB) used