Opening umbrellas indoor, was supposed to bring bad luck as in olden times the umbrella was supposed to be used only as a protection against the sun. Hence, opening it indoors was considered as a mark of disrespect to the sun god.
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No Jiro's post #7 makes the most sense to me.No, I think TXG's and mine are the most sensible....
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No, no bad luck unless a tiny thunderstorm cloud rages on you indoors.
The only time it's bad luck is if you knock over Ming vases when you open up your umbrella.