Lots.
I use both 3e and 1e style multiclassing, my rangers have wizarrd and druid spells, the bard gets druid spells, fly has a random duration and no parachute, fort save in place of ressurection survival and system shock, old school energy drain, the 1e haste spell (yes, complete with the aging effect), ghosts still age people, no magic item creation feats except brew potion and scribe scroll, no magic item shops for that matter.