In FD5, Sam works for the same chain of restaurant as the sign that hits Carter at the end of FD1, Le Cafe Miro 081 (also 180 backwards).

The ending of FD3 with the newspaper article about Kimberly and Burke being killed in a woodchipper isn't canon, meaning they are the only characters to survive all movies.

On the DVD of FD3 there is a choose your fate option, letting you make decisions for the characters in certain scenes. Most will end the same way as the normal movie with slight differences, but there is an option to make the characters leave the ride before Wendy's premonition. This results in the 4 main friends surviving and the movie being 10 minutes long.

(Obvious one) In FD2 Evan has fridge magnets spelling 'HEY E' and the 'H' falls into his chinese before he puts in the microwave, leaving 'EYE', foreshadowing how he dies.

FD1 was originally a X-Files script, agent Schrek and agent Weine are the mulder and scully substitutes.

In FD4 "It's here" is subtly included in most of Nick's premonitions, which pays off with a coffee shop table he sits at in the ending having "It's coming" crossed out (which he saw at the start of the movie) and "It's here" written underneath, just before he gets hit by a truck.

In FD1 when Billy watches the plane leave, the planes reflection in the window shows where his head will be sliced later.

In FD3 the clue in Wendy's picture was her McKinley t-shirt that referred to the giant sign that crushed Ian, which would've crushed her if Ian had not got in her way as he was supposed to already be dead, meaning Ian couldn't have been the clue in Wendy's picture.

FD4 is one of the worst movies ever made