Walking under a ladder
Although a little vague in documentation most sources of this superstition agree the person that walks under a ladder is breaking a sacred triangle between the ladder, the wall and the floor. This triangle can represent various divine symbols such as the religious holy trinity, or the animus(soul). There is another theory it reminds people of the ladder on the gallows, and anything to do with gallows has always been bad luck. Of course its also unadvisable practically as whatever is up the ladder may fall on your head.