Back in college, my suitemates decided to prank a friend of ours who lived on the floor above us. During an entire Sunday afternoon, 8 of us blew up balloons which we stored in one of our bedrooms, and once our friend went to work on Monday evening (at the campus library), we filled her room from floor to ceiling with the balloons.
Ironically, as a relatively direct result of this prank, she and I began dating. She mentioned (playfully) that she would “get me back” for the balloon prank, but that I would probably never realize what she had done.
Her revenge was so subtle that I, indeed, had no idea that anything had been done to create the minor annoyance that plagued me for the remainder of our time at college. She sewed a library book security tag into the inside of my backpack, so every time I left the library, I set off the alarm and had my backpack searched.