Despite how regulated the film industry is, movie sets are still chaotic, unpredictable affairs. Everyone from the lead actors to the grip department are expected to work long hours starting early in morning and going to late at night, some scenes can only be filmed at certain times and locations which need to be organized in advanced, and random acts of nature can delay shooting, costing everyone time and money.
Some days, the only thing keeping the director from running his cast and crew into the ground are union rules. However, on-set incidents don't necessarily hinder a production. The spontaneity of happenstance has often lent itself to accidentally creating a number of unexpected but not unappreciated film moments, some of which ended up reshaping the entire film.
Sometimes an actor's demands would force the crew to improvise a different solution to a scene, sometimes mechanical or set malfunctions caused actors to realize and embrace a new element of their character, and sometimes an accident that should have ruined a take instead let the filmmakers explore the limits of their medium. But when accidents have a beneficial impact on a film, it can lead to some of the most iconic shots in the history of cinema.