6. Every Movie With A Phone, Ever
The Myth: 555 isn't a real phone number. You cant have helped but notice every time a character gives a phone number in a move, it always has a 555 slap bang in the middle. The reason for this is painfully simple (and well known). 555 is a sequence of numbers that works as either an area code, or start of a number in films that doesn't exist in the real world. If real numbers were used, the unlucky people would end up with countless prank calls of varying meanness, depending on who their number had being assigned to. It'd be like Tony Stark giving away your home address instead of his own to the Mandarin in Iron Man 3.
The Shocking Truth: They do exist. In the nineties, a change was made so that the only fake, fictional numbers were 555-0100 to 0199; all others were available to be used. But while that means 555 numbers do exist on a technicality, the real fact is much more interesting. 555 numbers are only restricted for fictional use in the US, making for a terrible misunderstanding. On one panel of The Far Side, a surrealist comic from the late eighties and nineties, a graphitised number was allocated to Satan. Unfortunately, it just so happened that number actually existed, owned by someone in Austrakua. Not directly film, but it shows the influence myths can have.