Try to watch films keeping an eye and ear out for every reference to a character's past. You will find plenty, even in totally original standalone films. Because that's what you do, you give a character a backstory that took place well before the events of the film.
There is no difference to the audience between a reference to a completely imaginary event, and a reference to a previously filmed event in a TV episode. Also no difference to a real event. When someone references the Lincoln assassination, there will of course be people who don't know who Lincoln was. Should that be a reason to not make a reference when the story requires it?