I tried to put a command block in the horse chest. It crashed.
---- Minecraft Crash Report ----
// Ooh. Shiny.

Time: 1/11/14 8:18 PM
Description: Updating screen events

java.lang.ArrayIndexOutOfBoundsException: 2
at wa.a(SourceFile:115)
at ajk.d(SourceFile:81)
at aib.a(SourceFile:171)
at cem.a(SourceFile:368)
at byl.a(SourceFile:515)
at byl.b(SourceFile:477)
at bxf.k(SourceFile:411)
at bxf.p(SourceFile:382)
at bsu.r(SourceFile:1473)
at bsu.as(SourceFile:877)
at bsu.a(SourceFile:314)
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 wa.a(SourceFile:115)
at ajk.d(SourceFile:81)
at aib.a(SourceFile:171)
at cem.a(SourceFile:368)
at byl.a(SourceFile:515)
at byl.b(SourceFile:477)
at bxf.k(SourceFile:411)
at bxf.p(SourceFile:382)

– Affected screen –
Details:
Screen name: bzi

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [cio['DudeTDM'/27, l='MpServer', x=-64.94, y=64.85, z=313.31]]
Chunk stats: MultiplayerChunkCache: 272, 272
Level seed: 0
Level generator: ID 04 - customized, ver 0. Features enabled: false
Level generator options:
Level spawn location: -244.00,64.00,200.00 - World: (-244,64,200), Chunk: (at 12,4,8 in -16,12; contains blocks -256,0,192 to -241,255,207), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 40263 game time, 40263 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: 27 total; [aep['Creeper'/64, l='MpServer', x=-40.00, y=64.00, z=290.53], abt['Zombie Horse'/65, l='MpServer', x=-44.56, y=71.00, z=358.81], aep['Creeper'/66, l='MpServer', x=-37.50, y=74.00, z=382.50], age['Spider'/11, l='MpServer', x=-91.50, y=63.00, z=380.50], agj['Zombie'/75, l='MpServer', x=-31.50, y=76.00, z=319.50], afw['Skeleton'/85, l='MpServer', x=-13.50, y=74.00, z=323.50], age['Spider'/84, l='MpServer', x=-0.50, y=74.00, z=313.50], aep['Creeper'/16, l='MpServer', x=-74.03, y=62.58, z=308.22], cio['DudeTDM'/27, l='MpServer', x=-64.94, y=64.85, z=313.31], aep['Creeper'/83, l='MpServer', x=-8.50, y=74.00, z=315.50], age['Spider'/23, l='MpServer', x=-62.31, y=69.91, z=291.97], abt['Zombie Horse'/25, l='MpServer', x=-61.91, y=73.00, z=345.28], abt['Mule'/24, l='MpServer', x=-52.16, y=70.00, z=345.78], abt['Horse'/26, l='MpServer', x=-61.72, y=66.00, z=367.25], aca['Pig'/102, l='MpServer', x=11.50, y=69.00, z=374.50], aca['Pig'/103, l='MpServer', x=14.41, y=67.00, z=361.97], aep['Creeper'/237, l='MpServer', x=-25.56, y=72.00, z=349.34], age['Spider'/100, l='MpServer', x=13.69, y=63.91, z=347.75], aca['Pig'/101, l='MpServer', x=10.50, y=68.00, z=369.50], aep['Creeper'/98, l='MpServer', x=2.28, y=71.00, z=299.50], agj['Zombie'/232, l='MpServer', x=-69.66, y=72.00, z=367.34], agj['Zombie'/97, l='MpServer', x=3.16, y=71.00, z=299.44], aep['Creeper'/163, l='MpServer', x=-80.00, y=73.00, z=371.59], aep['Creeper'/162, l='MpServer', x=-89.69, y=73.00, z=362.47], aep['Creeper'/227, l='MpServer', x=-62.50, y=72.00, z=339.50], aca['Pig'/104, l='MpServer', x=13.19, y=68.00, z=372.78], abt['Horse'/126, l='MpServer', x=-64.94, y=64.00, z=313.31]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Integrated singleplayer server
Stacktrace:
at cen.a(SourceFile:308)
at bsu.b(SourceFile:2252)
at bsu.a(SourceFile:323)
at net.minecraft.client.main.Main.main(SourceFile:120)

– System Details –
Details:
Minecraft Version: 1.8
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_65, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 132995736 bytes (126 MB) / 354689024 bytes (338 MB) up to 523501568 bytes (499 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Launched Version: 1.8
LWJGL: 2.9.1
OpenGL: Mobile Intel(R) 4 Series Express Chipset Family GL version 2.1.0 - Build 8.15.10.2869, Intel
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because EXT_framebuffer_object is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: No
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)