Two players where just flying along in spectator mode at a reasonably quick pace.

It borked.

Not sure if these are reported via Hopper because dedicated server.

Edit: I don't see the head of the stacktrace in the crash report, so here it is... Yay threads!

java.util.ConcurrentModificationException
at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1115)
at java.util.TreeMap$KeyIterator.next(TreeMap.java:1169)
at qr.a(SourceFile:575)
at qr.a(SourceFile:555)
at bfr.a(SourceFile:258)
at bfr.a(SourceFile:113)
at qq.b(SourceFile:160)
at qq.a(SourceFile:203)
at qr.a(SourceFile:766)
at net.minecraft.server.MinecraftServer.a(SourceFile:359)
at net.minecraft.server.MinecraftServer.q(SourceFile:382)
at pc.run(SourceFile:714)