My GF lives in the country on acreage, and is very casual about locking doors, among other things.
Her Mom lived directly across from her, and would occasionally pop in unannounced, usually just to be nosy.
She (GF) started fucking an old school mate, and was letting him come to her house to fuck her.
I used to pray that her Mom would pop through the door and 'catch' her getting her pussy pounded. Especially if she was ass up for doggy style!
Unfortunately, or otherwise, that never happened, but it was a fun and enjoyable thought.