I put around 30,000 creepers through an end portal. I jumped in. I was getting about 0.1 fps. After a couple minutes, I heard a explosion. I got the out of memory screen. Then, I crashed:
--- BEGIN ERROR REPORT 9d472e94 --------
Full report at:
C:\Users\John\AppData\Roaming\.minecraft\crash-reports\crash-2013-03-09_22.55.43-server.txt
Please show that file to Mojang, NOT just this screen!
Generated 3/9/13 10:55 PM
-- Head --
Stacktrace:
at aqt.a(SourceFile:23)
at aou.b(SourceFile:567)
at aou.a(SourceFile:562)
at zv.a(SourceFile:935)
at mp.d(SourceFile:481)
at ng.e(SourceFile:977)
at ng.c(SourceFile:1185)
at rv.c(SourceFile:26)
at ng.l_(SourceFile:551)
at rv.l_(SourceFile:31)
at ro.l_(SourceFile:108)
at zv.a(SourceFile:1336)
at iz.a(SourceFile:508)
at zv.g(SourceFile:1314)
-- Entity being ticked --
Details:
Entity Type: Creeper (ro)
Entity ID: 2528962
Entity Name: Creeper
Entity's Exact location: 99.64, 50.36, 0.33
Entity's Block location: World: (99,50,0), Chunk: (at 3,3,0 in 6,0; contains blocks 96,0,0 to 111,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Entity's Momentum: -227.52, 1.26, 210.87
Stacktrace:
at zv.h(SourceFile:1221)
at iz.h(SourceFile:390)
-- Affected level --
Details:
Level name: Test World (Non Mob Spawners)
All players: 1 total; [jc['DeEpicness123'/730264, l='Test World (Non Mob Spawners)', x=100.00, y=49.00, z=0.00]]
Chunk stats: ServerChunkCache: 442 Drop: 0
Level seed: 516595084
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (0,0,0), Chunk: (at 0,0,0 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 0 game time, 0 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: ~~ERROR~~ NullPointerException: null
Stacktrace:
at net.minecraft.server.MinecraftServer.r(SourceFile:462)
at net.minecraft.server.MinecraftServer.q(SourceFile:397)
at bja.q(SourceFile:122)
at net.minecraft.server.MinecraftServer.run(SourceFile:331)
at gp.run(SourceFile:573)
-- System Details --
Details:
Minecraft Version: 1.5
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.6.0_31, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 56849664 bytes (54 MB) / 1041694720 bytes (993 MB) up to 1041694720 bytes (993 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 8835157 (494768792 bytes; 471 MB) allocated, 8835157 (494768792 bytes; 471 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
Profiler Position: N/A (disabled)
Vec3 Pool Size: 33 (1848 bytes; 0 MB) allocated, 17 (952 bytes; 0 MB) used
Player Count: 1 / 8; [jc['DeEpicness123'/730264, l='Test World (Non Mob Spawners)', x=100.00, y=49.00, z=0.00]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.
java.lang.OutOfMemoryError: Java heap space
at aqt.a(SourceFile:23)
at aou.b(SourceFile:567)
at aou.a(SourceFile:562)
at zv.a(SourceFile:935)
at mp.d(SourceFile:481)
at ng.e(SourceFile:977)
at ng.c(SourceFile:1185)
at rv.c(SourceFile:26)
at ng.l_(SourceFile:551)
at rv.l_(SourceFile:31)
at ro.l_(SourceFile:108)
at zv.a(SourceFile:1336)
at iz.a(SourceFile:508)
at zv.g(SourceFile:1314)
at zv.h(SourceFile:1221)
at iz.h(SourceFile:390)
at net.minecraft.server.MinecraftServer.r(SourceFile:462)
at net.minecraft.server.MinecraftServer.q(SourceFile:397)
at bja.q(SourceFile:122)
at net.minecraft.server.MinecraftServer.run(SourceFile:331)
at gp.run(SourceFile:573)
--- END ERROR REPORT 31ba3297 ----------
Linked issues
Comments

How about:
DON'T SPAWN SO MANY ENTITIES!
This is the user's fault. Minecraft should never have to handle so many entities.

"A full error report has been saved to C:\Users\USERNAME\AppData\Roaming\.minecraft\crash-reports\crash-CURRENTDATE.txt - Please include a copy of that file (Not this screen!) if you report this crash to anyone; without it, they will not be able to help fix the crash 😞"

If you make such experiments, assign more RAM to the JVM instead of the default 1GB.
That's an old version of Java, uninstall it and update to the latest Java 7.