You run out of toner or ink in the middle of a story, regardless of story length (and because with toner the print quality gradually fades you always have to run out the whole tale again).
Pens always vanish. It doesn't matter how many you have in the house or on your person. They all go.
Power cuts always happen in the middle of an important print run.
Ideas for new stories always come at the most awkward times (on the loo, in the shower, in church, basically anywhere where you can't whip out a notebook and commit the idea to paper).
You run out of stamps (or have problems with your email provider) the very moment you want to get a piece of work out and there's a deadline looming.