When I equip and un-equip the Maestro's Violin Bow item on my bard ((The item gives Bard Extra Spell Slot 1-2, as well as +2 Perform), it eats one of my spell slots. Everytime I do it, it destroys another one. It's an interesting bug, and with a bit of care can be avoided, however wanted to point it out. I don't know if it happens only on me or others too.
Recreate: Keep equipping and unequipping the item on a Bard, and watch the spell slots just disappear.