Crashing whenever i join a server

---- Minecraft Crash Report ----
// On the bright side, I bought you a teddy bear!

Time: 09/07/13 19:26
Description: Rendering entity in world

java.lang.ClassFormatError: Truncated class file
at java.lang.Character.toLowerCase(Unknown Source)
at java.lang.Character.toLowerCase(Unknown Source)
at java.lang.String.toLowerCase(Unknown Source)
at java.lang.String.toLowerCase(Unknown Source)
at avf.a(SourceFile:341)
at avf.b(SourceFile:479)
at avf.a(SourceFile:251)
at avf.b(SourceFile:236)
at bgy.a(SourceFile:432)
at bhg.a(SourceFile:326)
at bhg.a(SourceFile:24)
at bgy.b(SourceFile:372)
at bgy.a(SourceFile:207)
at bhg.a(SourceFile:122)
at bhg.a(SourceFile:24)
at bgi.a(SourceFile:204)
at bgi.a(SourceFile:184)
at bfi.a(SourceFile:326)
at bfb.a(SourceFile:845)
at bfb.b(SourceFile:693)
at ats.S(SourceFile:686)
at ats.d(SourceFile:619)
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 java.lang.Character.toLowerCase(Unknown Source)
at java.lang.Character.toLowerCase(Unknown Source)
at java.lang.String.toLowerCase(Unknown Source)
at java.lang.String.toLowerCase(Unknown Source)
at avf.a(SourceFile:341)
at avf.b(SourceFile:479)
at avf.a(SourceFile:251)
at avf.b(SourceFile:236)
at bgy.a(SourceFile:432)
at bhg.a(SourceFile:326)
at bhg.a(SourceFile:24)
at bgy.b(SourceFile:372)
at bgy.a(SourceFile:207)
at bhg.a(SourceFile:122)
at bhg.a(SourceFile:24)

– Entity being rendered –
Details:
Entity Type: null (bev)
Entity ID: 6417162
Entity Name: ottewillyy
Entity's Exact location: -514.21, 64.46, 1923.88
Entity's Block location: World: (-515,64,1923), Chunk: (at 13,4,3 in -33,120; contains blocks -528,0,1920 to -513,255,1935), Region: (-2,3; contains chunks -64,96 to -33,127, blocks -1024,0,1536 to -513,255,2047)
Entity's Momentum: 0.00, 0.00, 0.00

– Renderer details –
Details:
Assigned renderer: bhg@1a6eeab
Location: -9.47,-1.67,2.22 - World: (-10,-2,2), Chunk: (at 6,-1,2 in -1,0; contains blocks -16,0,0 to -1,255,15), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Rotation: -91.40625
Delta: 0.5151706
Stacktrace:
at bgi.a(SourceFile:204)
at bgi.a(SourceFile:184)
at bfi.a(SourceFile:326)
at bfb.a(SourceFile:845)

– Affected level –
Details:
Level name: MpServer
All players: 2 total; [bdf['drunken159'/6558800, l='MpServer', x=-504.80, y=66.12, z=1921.65], bev['ottewillyy'/6417162, l='MpServer', x=-514.21, y=64.46, z=1923.88]]
Chunk stats: MultiplayerChunkCache: 441
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (75,47,408), Chunk: (at 11,2,8 in 4,25; contains blocks 64,0,400 to 79,255,415), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 16266220 game time, 23139264 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: survival (ID 0). Hardcore: false. Cheats: false
Forced entities: 46 total; [bdf['drunken159'/6558800, l='MpServer', x=-504.80, y=66.12, z=1921.65], od['entity.LeashKnot.name'/6540144, l='MpServer', x=-484.50, y=64.50, z=1929.50], od['entity.LeashKnot.name'/6508896, l='MpServer', x=-484.50, y=64.50, z=1913.50], rr['Horse'/6493019, l='MpServer', x=-481.56, y=63.00, z=1914.34], rr['Horse'/6493018, l='MpServer', x=-485.75, y=63.00, z=1915.84], ry['Sheep'/-2147483647, l='MpServer', x=-8213.32, y=98.55, z=347.85], rr['Horse'/6493017, l='MpServer', x=-497.63, y=64.00, z=1916.25], ts['Spider'/6559487, l='MpServer', x=-454.50, y=17.00, z=1896.50], tv['Zombie'/6559042, l='MpServer', x=-522.50, y=27.00, z=1874.50], rr['Horse'/6493021, l='MpServer', x=-489.09, y=63.00, z=1926.81], rr['Horse'/6493020, l='MpServer', x=-484.22, y=63.00, z=1927.44], bev['ottewillyy'/6417162, l='MpServer', x=-514.21, y=64.46, z=1923.88], rw['entity.Cat.name'/6493009, l='MpServer', x=-516.31, y=58.88, z=1931.31], rr['Horse'/6493013, l='MpServer', x=-496.72, y=64.00, z=1925.16], bev['ottewillyy'/6417162, l='MpServer', x=-534.63, y=64.50, z=1926.03], rr['Horse'/6493012, l='MpServer', x=-496.78, y=64.00, z=1928.03], rn['Bat'/6553168, l='MpServer', x=-505.44, y=32.03, z=1893.22], rr['Horse'/6539017, l='MpServer', x=-483.97, y=63.00, z=1928.84], od['entity.LeashKnot.name'/6541939, l='MpServer', x=-486.50, y=64.50, z=1929.50], od['entity.LeashKnot.name'/6518086, l='MpServer', x=-489.50, y=64.50, z=1913.50], rn['Bat'/6553194, l='MpServer', x=-515.75, y=33.10, z=1896.25], tq['Skeleton'/6556757, l='MpServer', x=-521.50, y=24.00, z=1897.50], rn['Bat'/6553195, l='MpServer', x=-535.16, y=34.86, z=1878.47], bev['ottewillyy'/6417162, l='MpServer', x=-514.21, y=64.46, z=1923.88], tv['Zombie'/6512957, l='MpServer', x=-535.50, y=39.00, z=1881.50], rx['Pig'/6493055, l='MpServer', x=-512.53, y=31.00, z=1879.47], tq['Skeleton'/6553706, l='MpServer', x=-516.50, y=37.00, z=1875.50], tq['Skeleton'/6493056, l='MpServer', x=-527.50, y=41.00, z=1880.50], od['entity.LeashKnot.name'/6493071, l='MpServer', x=-499.50, y=65.50, z=1929.50], od['entity.LeashKnot.name'/6493072, l='MpServer', x=-499.50, y=65.50, z=1924.50], rn['Bat'/6521376, l='MpServer', x=-516.28, y=37.00, z=1879.63], rr['Horse'/6507432, l='MpServer', x=-486.56, y=63.00, z=1914.22], tq['Skeleton'/6559411, l='MpServer', x=-532.50, y=35.00, z=1876.50], od['entity.LeashKnot.name'/6515013, l='MpServer', x=-499.50, y=65.50, z=1917.50], te['Creeper'/6557106, l='MpServer', x=-537.50, y=38.00, z=1882.50], tq['Skeleton'/6559412, l='MpServer', x=-531.50, y=35.00, z=1875.50], od['entity.LeashKnot.name'/6540993, l='MpServer', x=-490.50, y=64.50, z=1929.50], tv['Zombie'/6521135, l='MpServer', x=-539.50, y=36.00, z=1884.50], te['Creeper'/6559491, l='MpServer', x=-530.50, y=37.00, z=1893.50], tq['Skeleton'/6530090, l='MpServer', x=-528.50, y=35.00, z=1942.50], od['entity.LeashKnot.name'/6509637, l='MpServer', x=-486.50, y=64.50, z=1913.50], ts['Spider'/6535491, l='MpServer', x=-530.50, y=30.00, z=1879.50], tq['Skeleton'/6548416, l='MpServer', x=-548.50, y=29.00, z=1940.50], te['Creeper'/6557073, l='MpServer', x=-508.63, y=37.00, z=1906.28], te['Creeper'/6557074, l='MpServer', x=-506.56, y=38.00, z=1904.84], tv['Zombie'/6554062, l='MpServer', x=-461.50, y=25.00, z=1880.50]]
Retry entities: 0 total; []
Server brand: