This Joke Wasn’t an Accident
Professor Utonium of the Powerpuff Girls had the power to create the perfect little girl. He mixed three ingredients: sugar, spice, and everything nice. However, accidentally adding chemical X to the mixture made the little heroes everyone loves.
Source: Facebook
In one of the episodes, Bubbles goes to the professor with Robin, her new friend. After a while, it dawns on the young girl that the professor accidentally mixed chemicals and made the PPG. She then turns to Professor Utonium and tells him not to worry because she was an accident too. This was too hard to interpret as a kid, but you probably can get the joke now.