Not entirely sure if it's able to be recreated, or how rare it is, but this happened.
I went to the Enchantress to upgrade a legendary bow I had in the public hub. I used 2 other legendary items and 3 mythical items, none of which had any use to me. I got great success, giving double points. While it was adding all the exp to my bow, it reached a certain level, somewhere close to level 10, and the noise for adding exp kept playing even when it should have stopped. On top of that, when it should have been playing, there were 2 versions of it, one that stopped and restarted when it was supposed to, and one that continued indefinitely. I tried leaving the Enchantress immediately after it stopped leveling up (Got 17 levels on it by the way, go me) and it continued playing until I went to my own private tavern and then went back.
Obviously this is not a game-breaking bug of any kind, and it can be fixed by just going to a different area, so it's not a big deal, but when it happens it hurts the ears.