O wow, I actually know where this "myth" comes from, and I simply have to share.
Centuries ago, mirrors were made by hand. The better the quality of the mirror, the higher it's price... and the rich were determined to have good quality reflective glass, not distorted imagery. It got to the point that one mirror of any size was worth as much (if not more) than a horse team and carriage: in today's terms, that's like paying the same for a mirror as you do for a 'newer' used car!! Orders placed with glass artisans could take years to fill, and even with such a steep price tag, the wealthy queued up to buy from the trendiest and best mirror-makers.
Those same folks had maids and housekeeping staff, and after spending such an insane amount of money on a mirror, the lords and ladies were determined to make sure the staff were extra careful with their prized mirrors. "Seven years of bad luck", they said, using the number of sons that Job had in the Bible as the inkling of "proof" in those words.... and the uneducated and superstitious that were employed as house servants were suitably freaked out, and very very careful about cleaning their master's mirrors. It must be true, after all, if there was a biblical reference behind what was being said, right?
It was a case of the wealthy trying to protect a major investment from the clumsiness of their cleaning staff.... and when you've spent the equivalent of $20,000 in today's times on a piece of hand-made reflective glass, you're going to be a little, um, possessive of the investment.
Mirrors are now made by machines, and so don't have the cost that they once did.... but the 'rumor' that was started centuries ago persists, embroidered into the fabric of our society.