This happened while I was experimenting with different XP values for killing mobs, minibosses and bosses. If you kill an entity that gives more weapon XP than is required to reach the configured maximum level, the level of that weapon will sometimes overshoot past its maximum value. This also happens with armor. If this is intended, which I'm assuming it is not, it would be nice to at least have an option to disable this behavior.

Also, as a suggestion, it would be pretty useful if there was some way you could individually configure XP values from killing entities.