Great question! I was told both were bad luck as a kid and just not to do them or bad things would happen to me. Now that you have asked, I looked it up for more detail. Walking under the ladder can anger the spirits (per Egyptian lore) or bring death (per Middle Age Christianity lore). For the umbrella one, it will just bring bad luck. Or according to the How Stuff Works website the Victorians were just worried about safety and the Ancient Egyptians did not want to anger the sun god since umbrellas there were meant for shade more than rain.