server stops working at random times

sometimes me and my friend are on my server, when all of a sudden i get this error

Code:

06-05 16:26:30 [SEVERE] java.lang.NullPointerException
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.EntityTrackerEntry.b(EntityTrackerEntry.java:251)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.EntityTrackerEntry.b(EntityTrackerEntry.java:170)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.EntityTrackerEntry.scanPlayers(EntityTrackerEntry.java:200)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.EntityTrackerEntry.track(EntityTrackerEntry.java:59)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.EntityTracker.a(EntityTracker.java:116)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:406)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:311)
2011-06-05 16:26:30 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-06-05 16:26:30 [SEVERE] Unexpected exception
java.lang.NullPointerException
    at net.minecraft.server.EntityTrackerEntry.b(EntityTrackerEntry.java:251)
    at net.minecraft.server.EntityTrackerEntry.b(EntityTrackerEntry.java:170)
    at net.minecraft.server.EntityTrackerEntry.scanPlayers(EntityTrackerEntry.java:200)
    at net.minecraft.server.EntityTrackerEntry.track(EntityTrackerEntry.java:59)
    at net.minecraft.server.EntityTracker.a(EntityTracker.java:116)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:406)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:311)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)

before this there is usually some sort of warning before it stops working like not being able to get out of a minecart