I had the same thing happen to me I hope it gets fixed soon I've been tempted to quit playing because this has become so irritating.

---- Minecraft Crash Report ----
// Ooh. Shiny.

Time: 7/5/13 1:27 AM
Description: Rendering screen

java.lang.OutOfMemoryError: unable to create new native thread
at java.lang.Thread.start0(Native Method)
at java.lang.Thread.start(Unknown Source)
at net.minecraft.server.MinecraftServer.v(SourceFile:579)
at atn.a(SourceFile:1426)
at avy.e(SourceFile:157)
at avz.a(SourceFile:210)
at avx.a(SourceFile:173)
at avy.a(SourceFile:183)
at bet.b(SourceFile:718)
at atn.S(SourceFile:685)
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 java.lang.Thread.start0(Native Method)
at java.lang.Thread.start(Unknown Source)
at net.minecraft.server.MinecraftServer.v(SourceFile:579)
at atn.a(SourceFile:1426)
at avy.e(SourceFile:157)
at avz.a(SourceFile:210)
at avx.a(SourceFile:173)
at avy.a(SourceFile:183)

– Screen render details –
Details:
Screen name: bli
Mouse location: Scaled: (189, 102). Absolute: (567, 593)
Screen size: Scaled: (534, 300). Absolute: (1600, 900). Scale factor of 3
Stacktrace:
at bet.b(SourceFile:718)
at atn.S(SourceFile:685)
at atn.d(SourceFile:618)
at net.minecraft.client.main.Main.main(SourceFile:101)

– System Details –
Details:
Minecraft Version: 1.6.1
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 264695936 bytes (252 MB) / 440471552 bytes (420 MB) up to 1037959168 bytes (989 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: 3, tallocated: 63
Launched Version: 1.6.1
LWJGL: 2.9.0
OpenGL: Intel(R) HD Graphics 3000 GL version 3.1.0 - Build 9.17.10.3062, Intel
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