Crash Report Attached

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

Time: 7/7/13 8:32 PM
Description: Updating screen events

java.lang.RuntimeException: Failed to check session lock, aborting
at alj.h(SourceFile:44)
at alj.<init>(SourceFile:30)
at alf.<init>(SourceFile:16)
at alg.a(SourceFile:83)
at atn.a(SourceFile:1411)
at aus.a(SourceFile:202)
at avv.a(SourceFile:71)
at aus.a(SourceFile:302)
at avv.d(SourceFile:125)
at avv.m(SourceFile:109)
at atn.k(SourceFile:1157)
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 alj.h(SourceFile:44)
at alj.<init>(SourceFile:30)
at alf.<init>(SourceFile:16)
at alg.a(SourceFile:83)
at atn.a(SourceFile:1411)
at aus.a(SourceFile:202)
at avv.a(SourceFile:71)
at aus.a(SourceFile:302)
at avv.d(SourceFile:125)
at avv.m(SourceFile:109)

– Affected screen –
Details:
Screen name: bli
Stacktrace:
at atn.k(SourceFile:1157)
at atn.S(SourceFile:662)
at atn.d(SourceFile:618)
at net.minecraft.client.main.Main.main(SourceFile:101)

– System Details –
Details:
Minecraft Version: 1.6.1
Operating System: Mac OS X (x86_64) version 10.7.5
Java Version: 1.6.0_51, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 63648600 bytes (60 MB) / 93917184 bytes (89 MB) up to 1065025536 bytes (1015 MB)
JVM Flags: 1 total; -Xmx1G
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 1.6.1
LWJGL: 2.9.0
OpenGL: AMD Radeon HD 6770M OpenGL Engine GL version 2.1 ATI-7.32.12, ATI Technologies Inc.
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: ~_ERROR~ NullPointerException: null