Open an umbrella inside and bad luck will "rain" on you.
Why do so many people believe this? Maybe it's something parents tell there children so they do not play with umbrella inside and break or poke their eyes out! or maybe it is one of these two ancient ideas:
The most common stems from the days when umbrellas were used mainly as protection against the sun. To open one indoors would be to insult the sun god and invite his wrath on everyone in your household.
Another theory borrows from the idea of an umbrella as a protector against the storms of life. If you were to open one in your home, the household guardian spirits might think you felt their protection was insufficient, and then they'd leave in a huff. Then everyone in the is cursed.