I was making a map in the newest snapshot and I needed a ladder, so I just got one out of my inventory and placed it. My game crashed.
If you want to re-create this, just place a ladder.
Here is the crash report:
---- Minecraft Crash Report ----
// Don't be sad. I'll do better next time, I promise!

Time: 7/10/14 12:17 AM
Description: Unexpected error

java.lang.IllegalArgumentException: Cannot get property bck

as it does not exist!
at bbw.b(SourceFile:91)
at avi.a(SourceFile:47)
at avi.a(SourceFile:35)
at apm.a(SourceFile:2320)
at ait.a(SourceFile:106)
at cdt.a(SourceFile:295)
at bqi.ar(SourceFile:1325)
at bqi.q(SourceFile:1682)
at bqi.ap(SourceFile:863)
at bqi.a(SourceFile:303)
at net.minecraft.client.main.Main.main(SourceFile:120)

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

– Head –
Stacktrace:
at bbw.b(SourceFile:91)
at avi.a(SourceFile:47)
at avi.a(SourceFile:35)
at apm.a(SourceFile:2320)
at ait.a(SourceFile:106)
at cdt.a(SourceFile:295)
at bqi.ar(SourceFile:1325)

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [chv['MAXXXX91'/0, l='MpServer', x=-1034.94, y=57.00, z=407.61]]
Chunk stats: MultiplayerChunkCache: 624, 624
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: -1027.00,4.00,514.00 - World: (-1027,4,514), Chunk: (at 13,0,2 in -65,32; contains blocks -1040,0,512 to -1025,255,527), Region: (-3,1; contains chunks -96,32 to -65,63, blocks -1536,0,512 to -1025,255,1023)
Level time: 387826 game time, 6000 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: false
Forced entities: 1 total; [chv['MAXXXX91'/0, l='MpServer', x=-1034.94, y=57.00, z=407.61]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Integrated singleplayer server
Stacktrace:
at cdu.a(SourceFile:308)
at bqi.b(SourceFile:2139)
at bqi.a(SourceFile:317)
at net.minecraft.client.main.Main.main(SourceFile:120)

– System Details –
Details:
Minecraft Version: 14w28a
Operating System: Windows Vista (amd64) version 6.0
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 105481216 bytes (100 MB) / 479133696 bytes (456 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
Launched Version: 14w28a
LWJGL: 2.9.1
OpenGL: AMD Radeon HD 6800 Series GL version 4.2.12002 Compatibility Profile Context 9.12.0.0, ATI Technologies Inc.
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.

Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: []
Current Language: English (US)
Profiler Position: N/A (disabled)