Here, I'll copy and paste the crash/error when I log into a MC server I play on.

Time: 11/5/13 8:41 PM
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
at abp.a(SourceFile:331)
at xj.k(SourceFile:203)
at xl.e(SourceFile:430)
at blc.e(SourceFile:205)
at rh.h(SourceFile:1205)
at xl.h(SourceFile:240)
at bje.h(SourceFile:63)
at afn.a(SourceFile:1368)
at afn.g(SourceFile:1346)
at afn.h(SourceFile:1253)
at azd.o(SourceFile:1532)
at azd.ad(SourceFile:753)
at azd.e(SourceFile:704)
at net.minecraft.client.main.Main.main(SourceFile:103)

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

– Head –
Stacktrace:
at abp.a(SourceFile:331)
at xj.k(SourceFile:203)
at xl.e(SourceFile:430)
at blc.e(SourceFile:205)
at rh.h(SourceFile:1205)
at xl.h(SourceFile:240)
at bje.h(SourceFile:63)
at afn.a(SourceFile:1368)
at afn.g(SourceFile:1346)

– Entity being ticked –
Details:
Entity Type: null (bje)
Entity ID: 5506575
Entity Name: so2315
Entity's Exact location: 0.00, 254.62, 0.00
Entity's Block location: World: (0,254,0), Chunk: (at 0,15,0 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)
Entity's Momentum: 0.00, 0.00, 0.00
Stacktrace:
at afn.h(SourceFile:1253)

– Affected level –
Details:
Level name: MpServer
All players: 4 total; [bje['so2315'/5506575, l='MpServer', x=0.00, y=254.62, z=0.00], bld['§agooloo123'/5477941, l='MpServer', x=8.69, y=250.00, z=1.02], bld['§aLordOfTheRando'/5286343, l='MpServer', x=-0.78, y=250.00, z=-2.59], bld['§ame6ashy6uy'/5495719, l='MpServer', x=-17.41, y=250.00, z=4.29]]
Chunk stats: MultiplayerChunkCache: 49, 49
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (0,250,0), Chunk: (at 0,15,0 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: 296546236 game time, 300001513 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: true), thunder time: 0 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Forced entities: 7 total; [bje['so2315'/5506575, l='MpServer', x=0.00, y=254.62, z=0.00], bld['§agooloo123'/5477941, l='MpServer', x=8.69, y=250.00, z=1.02], bld['§aLordOfTheRando'/5286343, l='MpServer', x=-0.78, y=250.00, z=-2.59], bld['§ame6ashy6uy'/5495719, l='MpServer', x=-17.41, y=250.00, z=4.29], bld['§ame6ashy6uy'/5495719, l='MpServer', x=-17.41, y=250.00, z=4.29], bld['§aLordOfTheRando'/5286343, l='MpServer', x=-0.78, y=250.00, z=-2.59], bld['§agooloo123'/5477941, l='MpServer', x=8.69, y=250.00, z=1.02]]
Retry entities: 0 total; []
Server brand: craftbukkit
Server type: Non-integrated multiplayer server
Stacktrace:
at biz.a(SourceFile:284)
at azd.b(SourceFile:1951)
at azd.e(SourceFile:713)
at net.minecraft.client.main.Main.main(SourceFile:103)

– System Details –
Details:
Minecraft Version: 1.7.2
Operating System: Windows XP (x86) version 5.1
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
Memory: 33175728 bytes (31 MB) / 87945216 bytes (83 MB) up to 518979584 bytes (494 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
AABB Pool Size: 8236 (461216 bytes; 0 MB) allocated, 5 (280 bytes; 0 MB) used
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 1.7.2
LWJGL: 2.9.0
OpenGL: GeForce 9500 GT/PCI/SSE2 GL version 3.3.0, NVIDIA Corporation
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: []
Current Language: English (US)
Profiler Position: N/A (disabled)
Vec3 Pool Size: 33 (1848 bytes; 0 MB) allocated, 17 (952 bytes; 0 MB) used
Anisotropic Filtering: Off (1)