I was in a 1x1 tunnel when doing this.
How to do this bug/glitch:
1. Break sand
2. After doing step one quickly place a sign before the sand falls then click done
3. If failed, re-do step 1&2

Crash Report:

— BEGIN ERROR REPORT cbd2c02f --------
Full report at:
C:\Documents and Settings\Parent\Application Data\.minecraft\crash-reports\crash-2013-01-31_18.27.50-client.txt
Please show that file to Mojang, NOT just this screen!

Generated 1/31/13 6:27 PM

– Head –
Stacktrace:
at apy.k_(SourceFile:114)
at ayr.a(SourceFile:52)
at awy.a(SourceFile:72)
at awy.d(SourceFile:127)
at awy.m(SourceFile:111)

– Affected screen –
Details:
Screen name: ayr

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [bcu['CaptainWoko'/1, l='MpServer', x=583.20, y=61.62, z=2057.39]]
Chunk stats: MultiplayerChunkCache: 441
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: World: (608,4,2050), Chunk: (at 0,0,2 in 38,128; contains blocks 608,0,2048 to 623,255,2063), Region: (1,4; contains chunks 32,128 to 63,159, blocks 512,0,2048 to 1023,255,2559)
Level time: 2548 game time, 2548 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: 2 total; [qt['Falling Block'/2596, l='MpServer', x=581.50, y=61.50, z=2057.50], bcu['CaptainWoko'/1, l='MpServer', x=583.20, y=61.62, z=2057.39]]
Retry entities: 0 total; []
Stacktrace:
at bcr.a(SourceFile:282)
at net.minecraft.client.Minecraft.b(SourceFile:1870)
at net.minecraft.client.Minecraft.run(SourceFile:535)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 13w05a
Operating System: Windows XP (x86) version 5.1
Java Version: 1.7.0_10, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 271655952 bytes (259 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 9254 (518224 bytes; 0 MB) allocated, 3 (168 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
LWJGL: 2.4.2
OpenGL: ATI Radeon 3100 Graphics GL version 2.1.7980 Release, ATI Technologies Inc.
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: 76 (4256 bytes; 0 MB) allocated, 27 (1512 bytes; 0 MB) used

java.lang.NullPointerException
at apy.k_(SourceFile:114)
at ayr.a(SourceFile:52)
at awy.a(SourceFile:72)
at awy.d(SourceFile:127)
at awy.m(SourceFile:111)
at net.minecraft.client.Minecraft.l(SourceFile:1078)
at net.minecraft.client.Minecraft.K(SourceFile:574)
at net.minecraft.client.Minecraft.run(SourceFile:526)
at java.lang.Thread.run(Unknown Source)
— END ERROR REPORT 1c94d566 ----------

What was in the crash report file:
---- Minecraft Crash Report ----
// Hey, that tickles! Hehehe!

Time: 1/31/13 6:27 PM
Description: Updating screen events

java.lang.NullPointerException
at apy.k_(SourceFile:114)
at ayr.a(SourceFile:52)
at awy.a(SourceFile:72)
at awy.d(SourceFile:127)
at awy.m(SourceFile:111)
at net.minecraft.client.Minecraft.l(SourceFile:1078)
at net.minecraft.client.Minecraft.K(SourceFile:574)
at net.minecraft.client.Minecraft.run(SourceFile:526)
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 apy.k_(SourceFile:114)
at ayr.a(SourceFile:52)
at awy.a(SourceFile:72)
at awy.d(SourceFile:127)
at awy.m(SourceFile:111)

– Affected screen –
Details:
Screen name: ayr

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [bcu['CaptainWoko'/1, l='MpServer', x=583.20, y=61.62, z=2057.39]]
Chunk stats: MultiplayerChunkCache: 441
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: World: (608,4,2050), Chunk: (at 0,0,2 in 38,128; contains blocks 608,0,2048 to 623,255,2063), Region: (1,4; contains chunks 32,128 to 63,159, blocks 512,0,2048 to 1023,255,2559)
Level time: 2548 game time, 2548 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: 2 total; [qt['Falling Block'/2596, l='MpServer', x=581.50, y=61.50, z=2057.50], bcu['CaptainWoko'/1, l='MpServer', x=583.20, y=61.62, z=2057.39]]
Retry entities: 0 total; []
Stacktrace:
at bcr.a(SourceFile:282)
at net.minecraft.client.Minecraft.b(SourceFile:1870)
at net.minecraft.client.Minecraft.run(SourceFile:535)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 13w05a
Operating System: Windows XP (x86) version 5.1
Java Version: 1.7.0_10, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 271655952 bytes (259 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 9254 (518224 bytes; 0 MB) allocated, 3 (168 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
LWJGL: 2.4.2
OpenGL: ATI Radeon 3100 Graphics GL version 2.1.7980 Release, ATI Technologies Inc.
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: 76 (4256 bytes; 0 MB) allocated, 27 (1512 bytes; 0 MB) used