(Alternate title: Shagogilbad, the infinite spear)
(This bug is a lot like
http://www.bay12games.com/forum/index.php?topic=7973.0)
My Adventurer has a finely-crafted Cacao Bin containing Shagogilbad, an artifact aluminum spear from my previous fortress in that world. If he (r)removes Shagogilbad and wields it in his hand, another Shagogilbad remains in the bin in his inventory. In fact, he can do this arbitrarily many times, wielding five or six Shagogilbads. Even better, he can throw Shagogilbad - it will stay both where it lands and in the bin, but destroy all other copies.
If Shagogilbad gets stuck in some fleshy target, he can pull out more - his inventory will recognize them all as 'stuck', but you can see only one stuck in the target. Throw it, and it'll disappear off the target, allowing him to pull a fresh Shagogilbad from the bin...
I have the save file and can upload it if someone tells me how / requests it. I once single-handedly held a Mayor's house against some enemy humans by staying on the top floor tossing Shagogilbads as people came in - infinite ammo makes for fun standoffs... I honestly didn't even think to report this as a bug for a long time since I thought it was so entertaining. In case anyone is interested, I remember exactly when / how I retrieved it from my last fortress, because of some very unusual circumstances surrounding the other artifacts - the same thing happened to a sapphire ring, except with a Jeweler's shop instead of a bin, but I won't start telling stories unless people ask.
Best part of this? "Shagogilbad" means "the Static Guile" - duplicitous yet stationary.