If you worry about this one nowadays, you might want to consider never leaving the house again. Think about it – tube stations, fire exits, clubs... all of these would be off limits to the hardened believer. It's hard to pinpoint the origin of this superstition, although it could be another one that falls into the common sense category. After all, back when staircases were narrow and uncarpeted, two people shoving past each other on their way to a medieval banquet or village hanging were unlikely to care if they knocked the other down the stairs. Because that's what people were like back then.