The subject of this article:
"Shit happens" is a popular observation that bad things happen sometimes for no reason at all. It was very popular on bumper stickers.
In the Forrest Gump film, Forrest is shown to have inspired the use of the phrase. While he is running across the country, an old man (named "Aging Hippie" in the script) approaches and asks if Forrest could think of a good slogan for his new bumper sticker business. Just then, Forrest steps into a large pile of dog feces, which surprises the old man. Forrest reacts just by saying "It happens," inspiring the phrase.