When leaving multiplayer, there's a small chance that this crash happens:

---- Minecraft Crash Report ----
// Uh... Did I do that?

Time: 12/28/12 5:50 PM
Description: Unexpected error

java.lang.NullPointerException
at xc.bA(Unknown Source)
at net.minecraft.client.Minecraft.aY(Unknown Source)
at net.minecraft.client.Minecraft.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

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

– System Details –
Details:
Minecraft Version: 1.4.6
Operating System: Windows 8 (x86) version 6.2
Java Version: 1.7.0_09, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 92345000 bytes (88 MB) / 160485376 bytes (153 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 1 total; -Xmx1024m
AABB Pool Size: 28 (1568 bytes; 0 MB) allocated, 2 (112 bytes; 0 MB) used
Suspicious classes: [net.minecraft.client.a]
IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
LWJGL: 2.4.2
OpenGL: AMD Radeon HD 6520G GL version 4.2.11774 Compatibility Profile Context, ATI Technologies Inc.
Is Modded: Very likely; Jar signature invalidated
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~_ERROR~ NullPointerException: null