If, while you walk under a ladder, someone drops something on you, it is very bad luck. However, some people won't walk under a ladder at any time, though they cannot explain why.
A long time ago, people did not walk under ladders for a couple of reasons. One reason involves death. Before the gallows were invented, murderers were hung from the top of a ladder, and when they died, it is said that their ghosts remained for a long time where they had fallen. This made ladders very unpopular places.
Another reason involves life. In many religions, the symbol of life was a triangle. When a ladder is leaned against a wall, it forms a triangle. When someone walked under the ladder it broke the triangle, disrupted life, and invited one of the worst kinds of trouble.
If you do walk under a ladder, you still have a chance to fix it by walking back through the ladder the other way. This will erase what happened and give you another chance.