Just had the same problem twice. crashed my server both times. When you break a dimensional anchor, server crashes...
Edit:
2014-02-04 20:45:23 [iNFO] [sTDERR] java.lang.AssertionError: unref with refcount already 0! chunk pos [13, 7]
2014-02-04 20:45:23 [iNFO] [sTDERR] at mods.immibis.chunkloader.data.WorldLoaderList.unrefChunk(WorldLoaderList.java:171)
2014-02-04 20:45:23 [iNFO] [sTDERR] at mods.immibis.chunkloader.data.WorldLoaderList.tick(WorldLoaderList.java:280)
2014-02-04 20:45:23 [iNFO] [sTDERR] at mods.immibis.chunkloader.DimensionalAnchors.onTickInGame(DimensionalAnchors.java:558)
2014-02-04 20:45:23 [iNFO] [sTDERR] at mods.immibis.core.api.porting.PortableBaseMod$1.tickStart(PortableBaseMod.java:22)
2014-02-04 20:45:23 [iNFO] [sTDERR] at cpw.mods.fml.common.FMLCommonHandler.tickStart(FMLCommonHandler.java:122)
2014-02-04 20:45:23 [iNFO] [sTDERR] at cpw.mods.fml.common.FMLCommonHandler.onPreServerTick(FMLCommonHandler.java:287)
2014-02-04 20:45:23 [iNFO] [sTDERR] at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:576)
2014-02-04 20:45:23 [iNFO] [sTDERR] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:484)
2014-02-04 20:45:23 [iNFO] [sTDERR] at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)
2014-02-04 20:45:23 [sEVERE] Encountered an unexpected exception AssertionError
java.lang.AssertionError: unref with refcount already 0! chunk pos [13, 7]
at mods.immibis.chunkloader.data.WorldLoaderList.unrefChunk(WorldLoaderList.java:171)
at mods.immibis.chunkloader.data.WorldLoaderList.tick(WorldLoaderList.java:280)
at mods.immibis.chunkloader.DimensionalAnchors.onTickInGame(DimensionalAnchors.java:558)
at mods.immibis.core.api.porting.PortableBaseMod$1.tickStart(PortableBaseMod.java:22)
at cpw.mods.fml.common.FMLCommonHandler.tickStart(FMLCommonHandler.java:122)
at cpw.mods.fml.common.FMLCommonHandler.onPreServerTick(FMLCommonHandler.java:287)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:576)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:484)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)