This is the crash report, i was in a mine and this is this the report:

---- Minecraft Crash Report ----
// Don't do that.

Time: 5/08/14 10:58 AM
Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$KeyIterator.next(Unknown Source)
at qi.a(SourceFile:163)
at net.minecraft.server.MinecraftServer.y(SourceFile:614)
at net.minecraft.server.MinecraftServer.x(SourceFile:530)
at cxc.x(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:446)
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: 14w31a
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.7.0_65, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 292260792 bytes (278 MB) / 781713408 bytes (745 MB) up to 831520768 bytes (793 MB)
JVM Flags: 1 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump
IntCache: cache: 0, tcache: 9, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qr['Capi2000'/2624677, l='Fuck you survival world', x=669.98, y=23.00, z=-1938.75]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.