Banana skin

A banana skin, figuratively, is potential hazard, problem or slip-up, from the allusion that old, rotting banana skins can be dangerous and cause a fall if trodden on. No doubt this actually happened to enough people for American vaudeville to take it up as a comic routine at the turn of the late 19th/early 20th century. Harold Lloyd used the stunt in the silent movie The Flirt in 1917. A few years later, Buster Keaton used the same visual gag in his film The High Sign (1923).