I lost a bulb in my HH once. The board that the transformer and fuse bank sits on it raised just enough for an 89 to roll under, and it's fixed in place, and open at the high end. Used my husband's camera snake to see where it went to pry it back out. Still worked, too lol.
My two worst fallen screws were one that got stuck in the area where the sling licker comes through the playfield - it was also under a ramp. The other one I was about ready to send the kids into the game with magnets....spent days looking for it. Came back to it one more time and it was stuck between the speaker mesh and the cabinet.
Lessons learned: Use a bright light. Always have a telescoping magnet. Wear pants with pockets and never hold in your hands more than what you are working on right then. Put the other screws and things in your pocket. Don't put the pants with pockets full of screws in the laundry that way.