h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I'm not sure how true this is but it came from my old IT teacher.

If you crease a cheque along the magetic writing near the bottom so that the magnetic ink is cracked the automated readers cannot clear the cheques but as the cheque is otherwise perfectly legitimate it has to go into manual clearing taking about two weeks as opposed to a couple of days with a normal cheque

You can also pierce that area with a pin. This is especially good if you do it to the area to the right of the existing writing. When the cheque in processed, the amount is printed in there by the automatic processing machine and I remember hearing that it smears if the paper has been pierced!