On my bard, Waldo, it appears that I am losing my bonus spellslots if I log out for any length of time. The normal ones are there, but any extra I get from items are gone.
If you have bonus slots and you log out / take of the item / or the item becomes unwearable (due to polymorph, etc) then you will lose your bonus spellslots. By equipping / unequipping you can get rid of all of your spell slots without even casting! Nothing that can be done about this I fear. Just a problem with sorcerers / bards.
Alright, I knew about the unequip issue, so I'll just change my gear set before I log.