The 1981 Cadillac El Dorado had a balancing problem which was corrected by installing a steel plate underneath the driver's seat. This saved Frank Lawrence "Lefty" Rosenthal from a car bomb attempt, which served as the inspiration for a scene in Casino.
Electronic novelty items that speak or sing while moving a part of them, such as singing fish. The fact that a fish on a plaque is flapping its tail to the music and lipsyncing is funny enough as it is to many people, but let the batteries get low and they get slowed and off-keyed when they try to move themselves. The effect is pretty funny.
Some synthesizers, like the Hing Hon EK-001, can sound like they're dying if they're low on batteries, too.
This happens with the old edutainment toy Speak n' Spell, but that's a Bad Bad Bug.
The Simon memory game would actually run faster (and its notes would play higher) if the batteries started to get weak, making for a more difficult challenge. However when the batteries were really weak, the device would no longer work properly and would simply become a light show.
Genetic mutations. They make species evolve into having useful new traits and create wild new species.
The Cowpox virus. Being affected by this benign disease makes one immune to the far deadlier smallpox. This observation and subsequent successful use of a preventive treatment against smallpox forms the basis of vaccination medical therapy.
The Doctor Who11th Doctor Sonic Screwdriver toy can make one of two noises when you press a button (it toggles between two different SSD sound effects). However, if you tap the button quickly, you can get it to emit a third, completely different noise. Don't do this too often though, because it can damage the noise making electronics.
