Crash on player join

[TruePikachu]

Some players on my server now seem to be crashing the server when they join, and I'm having no luck finding the source of the problem:

java.util.ConcurrentModificationException

at java.util.HashMap$HashIterator.nextEntry(Unknown Source)

at java.util.HashMap$KeyIterator.next(Unknown Source)

at ii.a(EntityTracker.java:237)

at net.minecraft.server.MinecraftServer.r(MinecraftServer.java:692)

at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:599)

at bdz.q(IntegratedServer.java:170)

at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:497)

at fy.run(SourceFile:849)

Any thoughts/information?

[TruePikachu]

Just got another one of the crashes, same trace. Moving the player through the player.dat didn't fix anything, I'm just deleting them from NEI and the player list and going to give them what they had.

The spot they spawned in didn't have anything odd about it. It was also in mid-air on the second crash.

I currently believe that this crash happens after gravity is applied to the velocity, but before position is adjusted by velocity. I had zeroed the velocity, and there was a slight downward velocity in the player.dat.

Their inventory was entirely of Vanilla items, and I don't suspect inventory itself to be the cause of the problem. I backed up the player.dat before deleting it, and can report values from it on request.

A previous theory I had on the crash was that it was related to a player intersecting an object, but that doesn't seem to be the cause, as moving the player up into the air didn't fix it this time.

[TruePikachu]

Very interesting incident just happened, server was lagging a lot (I think from a combo of misplaced wrathlamps and water in a quarry requesting tons of chunk updates), and one of the regulars (who also tends to be the most frequent crasher) joined, server crashed. I set a ban on both them and everyone else who was on the server at the time (reason: no join ATM), and apparently they tried connecting (with the ban in place), and the server crashed, despite the ban.

I have absolutely no idea how the heck this could have happened. Any thoughts?

[TruePikachu]

Right now the best theory I have is that the crasher (from post #3) is using Java 7 on OSX. We are going to at some point see if they get their ban message on Java 6 without crashing the server. If they do, I'll make sure to make mention of it here.

[TruePikachu]

Having the problem again, with a different user now, no ideas on how to proceed

[TruePikachu]

Now-crasher is using Java 7 on Win7, the _only_ lead I have is Java 7, gonna investigate online

[TruePikachu]

At last, it seems the problem is solved for now.

I (as in the server) was running an outdated version of Java. After updating it to the latest version, the issues stopped.

Likely of note is that the person who was just crashing the server had a newer version, so if someone has this problem, it might be from Java version, which is a solution I could not find.