There are 2 type of expired film:
a) In the box
The film is as you bought it lying in your drawer for the last x years while you always thought "I should use this one next time I go out". The next time happens and you think "hmm, if this film doesnt work anymore... maybe keep it for some experiments" (which never gonna happen). The problem, this expired stock grows if you were to greedy while buying stock last time !
b) In the camera
This effect happened always to my grandma or auntie which kept the film in her point-and-shoot camera from Christmas to Easter and the 60th birthday of uncle Jim 3 years later. But it happens to me because I have this collection of cameras (and I only collect camera that I can use). Luckily I have a little brown notebook where I take note of all films, date and locations. Close to the end of 2007 now, I do some inventory and found 5 film in various cameras. Think gonna go out today, if it ever stops to rain, and shoot them !