Here is a crash report.

---- Minecraft Crash Report ----
// Sorry

Time: 8/14/14 11:32 PM
Description: Executing command block

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at aqo.a(SourceFile:2230)
at ah.b(SourceFile:361)
at ab.a(SourceFile:49)
at apz.a(SourceFile:98)
at aui.c(SourceFile:53)
at qs.a(SourceFile:527)
at qs.c(SourceFile:199)
at net.minecraft.server.MinecraftServer.z(SourceFile:599)
at net.minecraft.server.MinecraftServer.y(SourceFile:531)
at cya.y(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:447)
at java.lang.Thread.run(Unknown Source)

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

– Head –
Stacktrace:
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at aqo.a(SourceFile:2230)
at ah.b(SourceFile:361)
at ab.a(SourceFile:49)

– Command to be executed –
Details:
Command: /execute @e[type=PrimedTnt] ~ ~ ~ summon PrimedTnt
Name: @
Stacktrace:
at apz.a(SourceFile:98)
at aui.c(SourceFile:53)

– Block being ticked –
Details:
Block: minecraft:command_block[triggered=true]
Block location: World: (-122,252,255), Chunk: (at 6,15,15 in -8,15; contains blocks -128,0,240 to -113,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Stacktrace:
at qs.a(SourceFile:527)
at qs.c(SourceFile:199)

– Affected level –
Details:
Level name: Primed TNT Summon Destruction
All players: 0 total; []
Chunk stats: ServerChunkCache: 352 Drop: 0
Level seed: -2750863427487606007
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: -116.00,64.00,256.00 - World: (-116,64,256), Chunk: (at 12,4,0 in -8,16; contains blocks -128,0,256 to -113,255,271), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 12608 game time, 12608 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 148818 (now: false), thunder time: 113803 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
at net.minecraft.server.MinecraftServer.z(SourceFile:599)
at net.minecraft.server.MinecraftServer.y(SourceFile:531)
at cya.y(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:447)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 14w33a
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_55, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 485681776 bytes (463 MB) / 765460480 bytes (730 MB) up to 5726797824 bytes (5461 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx6G
IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
Profiler Position: N/A (disabled)
Player Count: 0 / 8; []
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.