When i used the command "/summon LeashKnot ~ ~ ~" minecraft crashed, don't really know why.
If you need to read the crash report here it is:

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

Time: 14/02/14 15.50
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
at aja.E(SourceFile:189)
at aja.c(SourceFile:195)
at tg.e(SourceFile:112)
at te.l_(SourceFile:91)
at aja.a(SourceFile:1367)
at ob.a(SourceFile:528)
at aja.g(SourceFile:1345)
at aja.i(SourceFile:1250)
at ob.i(SourceFile:401)
at net.minecraft.server.MinecraftServer.v(SourceFile:537)
at net.minecraft.server.MinecraftServer.u(SourceFile:471)
at cbx.u(SourceFile:128)
at net.minecraft.server.MinecraftServer.run(SourceFile:387)
at mr.run(SourceFile:657)

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

– Head –
Stacktrace:
at aja.E(SourceFile:189)
at aja.c(SourceFile:195)
at tg.e(SourceFile:112)
at te.l_(SourceFile:91)
at aja.a(SourceFile:1367)
at ob.a(SourceFile:528)
at aja.g(SourceFile:1345)

– Entity being ticked –
Details:
Entity Type: LeashKnot (tg)
Entity ID: 11001
Entity Name: entity.LeashKnot.name
Entity's Exact location: -559,50, 4,00, 2195,50
Entity's Block location: -560,00,4,00,2195,00 - World: (-560,4,2195), Chunk: (at 0,0,3 in -35,137; contains blocks -560,0,2192 to -545,255,2207), Region: (-2,4; contains chunks -64,128 to -33,159, blocks -1024,0,2048 to -513,255,2559)
Entity's Momentum: 0,00, 0,00, 0,00
Entity's Rider: ~_ERROR~ NullPointerException: null
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aja.i(SourceFile:1250)
at ob.i(SourceFile:401)

– Affected level –
Details:
Level name: Prova SnapShots
All players: 1 total; [oe['XxLorexXDD'/10, l='Prova SnapShots', x=-559,78, y=4,00, z=2186,97]]
Chunk stats: ServerChunkCache: 952 Drop: 0
Level seed: 38443493374132670
Level generator: ID 01 - flat, ver 0. Features enabled: true
Level generator options:
Level spawn location: -262,00,4,00,2299,00 - World: (-262,4,2299), Chunk: (at 10,0,11 in -17,143; contains blocks -272,0,2288 to -257,255,2303), Region: (-1,4; contains chunks -32,128 to -1,159, blocks -512,0,2048 to -1,255,2559)
Level time: 69161 game time, 1000 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 82425 (now: false), thunder time: 71812 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
at net.minecraft.server.MinecraftServer.v(SourceFile:537)
at net.minecraft.server.MinecraftServer.u(SourceFile:471)
at cbx.u(SourceFile:128)
at net.minecraft.server.MinecraftServer.run(SourceFile:387)
at mr.run(SourceFile:657)

– System Details –
Details:
Minecraft Version: 14w07a
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_15, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 77285200 bytes (73 MB) / 280625152 bytes (267 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [oe['XxLorexXDD'/10, l='Prova SnapShots', x=-559,78, y=4,00, z=2186,97]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.