Bukkit/CraftBukkit — comment on commit 7e9f6ae12fabbf0f712b29f28f45cc76892f83a0
File: src/main/java/org/bukkit/craftbukkit/util/LongHashset.java line 130
By Zahl:

This should be wl.lock() (and wl.unlock() below), maybe even use synchronized in line 143, as add() does it too when assigning to outer[].