Not my little sister, but my fiancee told me just a few days ago about something her little sister did when they were both younger.
My father-in-law had a huge collection of VHS films. His younger daughter, meanwhile, had developed a fascination with bubbles in the bathtub. Specifically, how things seemed to disappear beneath them when placed in the bathwater. She also quite enjoyed many of her father's videos. So she cunningly decided to mix her two favourite pastimes by placing videos in the bathwater.
That in itself is mildly amusing. What makes it hilarious is that my mother-in-law, despite being in the lounge room and seeing her daughter grabbing VHS after VHS, didn't notice anything was wrong until my sister-in-law had destroyed upwards of 30 videocassettes, informing her mother that "videos all gone. Gone wiv bubbles."