I created a Superflat world with the preset "The Void". I went into Creative Mode, flew under the platform, and pressed F3 to check what my Y value was. The game froze for a second or two, and then crashed. Here's the crash report:
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---- Minecraft Crash Report ----
// Daisy, daisy...

Time: 9/10/15 5:30 PM
Description: Unexpected error

java.lang.ArrayIndexOutOfBoundsException: -1
at aqz.a(SourceFile:552)
at bap.b(SourceFile:152)
at bap.a(SourceFile:70)
at bap.a(SourceFile:53)
at bah.a(SourceFile:217)
at bkp.a(SourceFile:1036)
at azx.as(SourceFile:954)
at azx.a(SourceFile:377)
at net.minecraft.client.main.Main.main(SourceFile:125)

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

– Head –
Stacktrace:
at aqz.a(SourceFile:552)
at bap.b(SourceFile:152)
at bap.a(SourceFile:70)
at bap.a(SourceFile:53)
at bah.a(SourceFile:217)

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [bkc['RedRaptor5'/0, l='MpServer', x=-584.75, y=-9.54, z=1181.25]]
Chunk stats: MultiplayerChunkCache: 49, 49
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: World: (-588,4,1191), Chunk: (at 4,0,7 in -37,74; contains blocks -592,0,1184 to -577,255,1199), Region: (-2,2; contains chunks -64,64 to -33,95, blocks -1024,0,1024 to -513,255,1535)
Level time: 776 game time, 776 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; [bkc['RedRaptor5'/0, l='MpServer', x=-584.75, y=-9.54, z=1181.25]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Integrated singleplayer server
Stacktrace:
at bid.a(SourceFile:366)
at azx.b(SourceFile:2369)
at azx.a(SourceFile:391)
at net.minecraft.client.main.Main.main(SourceFile:125)

– System Details –
Details:
Minecraft Version: 15w37a
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 108927680 bytes (103 MB) / 232632320 bytes (221 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 15w37a
LWJGL: 2.9.4
OpenGL: Intel(R) HD Graphics 4400 GL version 4.3.0 - Build 10.18.15.4256, Intel
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.
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)
CPU: 4x Intel(R) Core(TM) i7-4500U CPU @ 1.80GHz
===================================================
Hopefully this can be fixed before 1.9 is officially released!