I worry about why I forgot to charge them before they left.
If you’re going to spread the confusion, don’t settle for half-measures. Change out the $20 for 80 quarters. 80 quarters slipped beneath 80 doors means 80 times the confusion! 
Midget hookers???
If someone slipped anything paper under our door, chances are the dog or cat would destroy it before either of us saw it. Especially the cat - he loved to tear up paper.
I would take the $20 and slide a note back outside the door saying “Thank you, the curse has been lifted”.
No, those are ‘convenient fun size’
In a big apartment building like this I’d probably be a little paranoid and wonder if there is something on the bill. Pick it up with tongs and wash it before I touched it.
Once I saw the twenty, the long-ingrained habit would probably kick in. I’d put on *Pour Some Sugar on Me *and do my strip routine.