Tested with Forge 515, 516, 517.

Other errors located in file. The one below has been persistent through most builds (still think it is a problem with optifine as well)

 

This occurs whenever you disconnect whilst moving things like a frame quarry (this occured in previous forge builds but after taking optifine out I didn't see the error persisting so I don't know [also happens whilst trying to d/c from a world]):

2013-01-28 20:56:07 [iNFO] [sTDERR] java.lang.IllegalArgumentException: Could not get provider type for dimension 11, does not exist
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:108)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at net.minecraftforge.common.DimensionManager.shouldLoadSpawn(DimensionManager.java:194)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at im.b(ChunkProviderServer.java:72)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at il.sendThisChunkToPlayer(PlayerInstance.java:99)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at ik.c(PlayerManager.java:296)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at gm.e(ServerConfigurationManager.java:232)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at iv.c(NetServerHandler.java:169)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at gm.r(ServerConfigurationManager.java:910)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at net.minecraft.server.MinecraftServer.k(MinecraftServer.java:402)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at bdz.k(IntegratedServer.java:278)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:550)
2013-01-28 20:56:07 [iNFO] [sTDERR] 	at fy.run(SourceFile:849)

 

ForgeModLoader-client-0:

http://www.mediafire.com/?fuy3v4l7ezedniy

One of your mods, lets say Mystcraft, is adding a dimension without properly syncing the provides to the client. Mod issue, not forge.

Yeah, I figured it was mystcraft causing that issue. I just wanted to make sure. Thanks Lex .