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Yep, your regular old forge crash. I was using Millenaire, no other mods installed. Suddenly, it shut down. Here's the error report.

---- Minecraft Crash Report ----
// My bad.

Time: 12/30/12 10:57 PM
Description: Unexpected error

java.lang.NoClassDefFoundError: aux
at net.minecraft.client.Minecraft.l(Minecraft.java:1753)
at net.minecraft.client.Minecraft.J(Minecraft.java:846)
at net.minecraft.client.Minecraft.run(Minecraft.java:771)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: aux
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:179)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 4 more
Caused by: java.lang.NullPointerException
at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:172)
... 6 more
A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Affected level --
Details:
Level name: MpServer
All players: 1 total; [ays['Tangle10'/281, l='MpServer', x=260.67, y=99.77, z=327.36]]
Chunk stats: MultiplayerChunkCache: 342
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (244,64,233), Chunk: (at 4,4,9 in 15,14; contains blocks 240,0,224 to 255,255,239), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 2469 game time, 2469 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: 93 total; [px['item.tile.sapling.oak'/818, l='MpServer', x=170.28, y=74.13, z=227.82], ow['Bat'/558, l='MpServer', x=250.16, y=49.00, z=189.25], ow['Bat'/557, l='MpServer', x=231.28, y=49.00, z=197.06], px['item.tile.sapling.oak'/555, l='MpServer', x=223.13, y=64.13, z=231.13], px['item.tile.sapling.oak'/284, l='MpServer', x=221.88, y=72.13, z=152.19], ow['Bat'/560, l='MpServer', x=239.06, y=48.00, z=192.66], px['item.item.apple'/517, l='MpServer', x=172.38, y=71.13, z=232.25], ow['Bat'/798, l='MpServer', x=236.75, y=26.00, z=178.53], ow['Bat'/43, l='MpServer', x=141.47, y=47.00, z=210.53], px['item.tile.sapling.oak'/288, l='MpServer', x=217.16, y=72.13, z=227.13], ow['Bat'/772, l='MpServer', x=204.44, y=35.00, z=132.56], ow['Bat'/49, l='MpServer', x=139.25, y=48.10, z=232.13], ays['Tangle10'/281, l='MpServer', x=260.67, y=99.77, z=327.36], px['item.tile.rail'/59, l='MpServer', x=147.22, y=33.13, z=236.59], px['item.tile.sapling.oak'/56, l='MpServer', x=152.91, y=68.13, z=167.16], px['item.tile.rail'/62, l='MpServer', x=153.88, y=38.13, z=252.88], px['item.tile.rail'/61, l='MpServer', x=153.88, y=38.13, z=250.13], px['item.tile.rail'/60, l='MpServer', x=149.56, y=38.13, z=250.13], pk['Wolf'/68, l='MpServer', x=153.97, y=65.00, z=278.91], pk['Wolf'/69, l='MpServer', x=155.50, y=65.00, z=278.50], pk['Wolf'/70, l='MpServer', x=162.38, y=67.00, z=272.69], px['item.tile.rail'/64, l='MpServer', x=156.13, y=34.13, z=246.81], px['item.tile.rail'/65, l='MpServer', x=157.81, y=34.13, z=243.78], px['item.tile.rail'/66, l='MpServer', x=158.63, y=34.13, z=243.94], ow['Bat'/67, l='MpServer', x=150.41, y=43.00, z=236.31], pk['Wolf'/76, l='MpServer', x=179.16, y=63.00, z=196.22], px['item.tile.gravel'/77, l='MpServer', x=164.78, y=44.13, z=232.13], px['item.tile.gravel'/78, l='MpServer', x=162.19, y=44.13, z=232.50], pk['Wolf'/75, l='MpServer', x=174.47, y=64.00, z=192.95], pk['Wolf'/81, l='MpServer', x=173.50, y=64.00, z=249.50], px['item.tile.sapling.oak'/80, l='MpServer', x=175.19, y=66.13, z=232.88], pk['Wolf'/83, l='MpServer', x=161.06, y=67.00, z=276.50], pk['Wolf'/82, l='MpServer', x=175.50, y=66.00, z=252.50], pk['Wolf'/93, l='MpServer', x=180.03, y=63.00, z=197.00], pk['Wolf'/92, l='MpServer', x=177.75, y=63.00, z=193.31], px['item.tile.sapling.oak'/95, l='MpServer', x=183.88, y=71.13, z=228.84], px['item.tile.sapling.oak'/94, l='MpServer', x=176.53, y=64.13, z=232.78], px['item.tile.sapling.oak'/91, l='MpServer', x=191.06, y=72.13, z=177.88], px['item.tile.sapling.oak'/90, l='MpServer', x=191.88, y=71.13, z=180.88], px['item.tile.sapling.oak'/102, l='MpServer', x=193.13, y=73.13, z=182.13], px['item.item.apple'/373, l='MpServer', x=191.16, y=70.13, z=212.91], pk['Wolf'/103, l='MpServer', x=199.07, y=63.00, z=214.47], px['item.tile.sapling.oak'/374, l='MpServer', x=217.13, y=72.13, z=238.03], px['item.tile.sapling.oak'/581, l='MpServer', x=170.69, y=73.13, z=229.31], pk['Wolf'/96, l='MpServer', x=177.16, y=66.00, z=255.47], px['item.tile.sapling.oak'/371, l='MpServer', x=182.56, y=63.13, z=225.28], pk['Wolf'/97, l='MpServer', x=185.13, y=64.00, z=256.47], px['item.tile.sapling.oak'/106, l='MpServer', x=195.41, y=70.13, z=213.25], px['item.tile.sapling.oak'/377, l='MpServer', x=253.25, y=74.13, z=175.88], pk['Wolf'/107, l='MpServer', x=192.50, y=64.00, z=227.50], pk['Wolf'/104, l='MpServer', x=201.50, y=64.00, z=227.22], pk['Wolf'/105, l='MpServer', x=199.69, y=64.00, z=218.66], ow['Bat'/117, l='MpServer', x=222.75, y=22.10, z=263.59], px['item.tile.sapling.oak'/116, l='MpServer', x=212.69, y=73.13, z=241.13], px['item.tile.sapling.oak'/115, l='MpServer', x=216.34, y=71.13, z=239.28], ow['Bat'/113, l='MpServer', x=229.56, y=32.10, z=157.75], px['item.tile.sapling.oak'/127, l='MpServer', x=235.84, y=74.13, z=276.78], pk['Wolf'/126, l='MpServer', x=231.84, y=64.00, z=206.53], pk['Wolf'/125, l='MpServer', x=219.66, y=65.00, z=212.78], pk['Wolf'/124, l='MpServer', x=233.22, y=66.00, z=224.50], pk['Wolf'/123, l='MpServer', x=235.44, y=64.00, z=204.84], px['item.tile.sapling.oak'/362, l='MpServer', x=253.34, y=74.13, z=179.13], px['item.tile.sapling.oak'/410, l='MpServer', x=215.13, y=72.13, z=229.75], px['item.item.apple'/141, l='MpServer', x=262.56, y=71.13, z=169.81], px['item.tile.sapling.oak'/414, l='MpServer', x=175.13, y=69.13, z=231.13], px['item.tile.sapling.oak'/143, l='MpServer', x=264.78, y=73.13, z=169.13], px['item.tile.sapling.oak'/142, l='MpServer', x=267.56, y=69.13, z=162.13], px['item.item.apple'/131, l='MpServer', x=252.84, y=74.13, z=224.31], px['item.tile.torch'/130, l='MpServer', x=247.94, y=69.13, z=236.28], px['item.item.apple'/404, l='MpServer', x=195.88, y=71.13, z=218.13], pk['Wolf'/155, l='MpServer', x=285.50, y=64.00, z=156.50], pk['Wolf'/156, l='MpServer', x=286.50, y=64.00, z=157.50], pk['Wolf'/157, l='MpServer', x=279.77, y=64.00, z=156.44], px['item.tile.sapling.oak'/696, l='MpServer', x=216.88, y=72.13, z=228.69], px['item.tile.sapling.oak'/144, l='MpServer', x=268.59, y=72.13, z=171.38], px['item.tile.sapling.oak'/145, l='MpServer', x=264.22, y=72.13, z=171.34], pk['Wolf'/169, l='MpServer', x=288.50, y=64.00, z=155.50], px['item.tile.sapling.oak'/444, l='MpServer', x=215.13, y=71.13, z=243.41], pk['Wolf'/172, l='MpServer', x=289.59, y=69.00, z=149.28], pk['Wolf'/163, l='MpServer', x=284.50, y=64.00, z=256.50], pk['Wolf'/162, l='MpServer', x=278.75, y=64.00, z=250.72], px['item.tile.sapling.oak'/644, l='MpServer', x=217.88, y=72.13, z=231.13], pk['Wolf'/161, l='MpServer', x=284.50, y=64.00, z=254.50], px['item.tile.sapling.oak'/436, l='MpServer', x=219.38, y=70.13, z=234.56], pk['Wolf'/180, l='MpServer', x=290.31, y=63.00, z=262.38], px['item.tile.sapling.oak'/464, l='MpServer', x=222.06, y=71.13, z=228.19], px['item.tile.sapling.oak'/465, l='MpServer', x=251.88, y=74.13, z=223.34], ow['Bat'/508, l='MpServer', x=252.38, y=10.00, z=176.55], px['item.tile.sapling.oak'/713, l='MpServer', x=183.53, y=63.13, z=221.81], px['item.tile.sapling.oak'/715, l='MpServer', x=236.13, y=74.13, z=275.84], px['item.tile.sapling.oak'/714, l='MpServer', x=185.88, y=73.13, z=226.09], ow['Bat'/507, l='MpServer', x=257.44, y=15.00, z=183.25], px['item.tile.sapling.oak'/732, l='MpServer', x=154.75, y=85.13, z=160.19]]
Retry entities: 0 total; []
Stacktrace:
at ayp.a(WorldClient.java:440)
at net.minecraft.client.Minecraft.b(Minecraft.java:2401)
at net.minecraft.client.Minecraft.run(Minecraft.java:794)
at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
Minecraft Version: 1.4.6
Operating System: Windows XP (x86) version 5.1
Java Version: 1.7.0_09, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 162209992 bytes (154 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 13232 (740992 bytes; 0 MB) allocated, 3 (168 bytes; 0 MB) used
Suspicious classes: FML and Forge are installed
IntCache: cache: 2, tcache: 0, allocated: 1, tallocated: 63
FML: MCP v7.25 FML v4.6.12.511 Minecraft Forge 6.5.0.471 4 mods loaded, 4 mods active
mcp [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available
FML [Forge Mod Loader] (coremods) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available
Forge [Minecraft Forge] (coremods) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available
Millenaire [Millénaire] (millenaire3.3.2.zip) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available
LWJGL: 2.4.2
OpenGL: ATI RADEON XPRESS 200 Series GL version 2.1.8543 Release, ATI Technologies Inc.
Is Modded: Definitely; Client brand changed to 'forge,fml'
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: 1772 (99232 bytes; 0 MB) allocated, 15 (840 bytes; 0 MB) used

That's what I did. Since you Cannot open .jar files I changed it to a zip and combined them.

.jar is just a compressed file, it can be opened.
And just so you're aware, aux.class is the class that handles the GUI for Achievements.