'Twas brillig, and the slithy toves did gyre and gimble in the wabe

It's in commercial code so I can't really post the actual code. In essence, there's a list of versioned items that one thread keeps track of. That thread populates the list with the existing items when it starts. Another thread uses this list to work whether an item exists (and what version it is), or if it needs to create the item. If it creates the item, it always creates the latest version of it (which has a different name to older versions). The list is fully synchronised, but the thread startup wasn't. This meant that you could get two of the same item but with different versions, which horribly confused code in a completely different module.