Players were doing their normal routine surviving when the server up and croaked on us. Giving us this 10 line error.
[16:57:03] [Server thread/ERROR]: Encountered an unexpected exception java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429) ~[?:1.8.0_66] at java.util.HashMap$KeyIterator.next(HashMap.java:1453) ~[?:1.8.0_66] at net.minecraft.server.v1_10_R1.EntityTracker.updatePlayers(EntityTracker.java:173) ~[custom18.jar:git-Bukkit-10c10b3] at net.minecraft.server.v1_10_R1.MinecraftServer.D(MinecraftServer.java:753) ~[custom18.jar:git-Bukkit-10c10b3] at net.minecraft.server.v1_10_R1.DedicatedServer.D(DedicatedServer.java:360) ~[custom18.jar:git-Bukkit-10c10b3] at net.minecraft.server.v1_10_R1.MinecraftServer.C(MinecraftServer.java:646) ~[custom18.jar:git-Bukkit-10c10b3] at net.minecraft.server.v1_10_R1.MinecraftServer.run(MinecraftServer.java:550) [custom18.jar:git-Bukkit-10c10b3] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_66]