(Fortune) -- You have to love the ways computers find new ways to screw up our lives, in matters big and small.
One day, a computer glitch at the New York Stock Exchange slices $1 trillion or so off the value of the U.S. stock market. Another day -- today, actually -- a computer glitch involving a somewhat smaller amount afflicts me when I tried to use my corporate flexible spending card for an 11-part purchase at my local drug store.
It's the same problem of computers talking to computers, without common sense being part of the programming. The difference is that the Short Hills Pharmacy is a customer-friendly retail shop run by a human being, who almost instantly diagnosed the problem and solved it. By contrast, because the NYSE now consists largely of computers trading with each other, the problem got way out of hand until human beings finally managed to intervene. By then, the financial world was trembling.
My problem turned out to be that I was using a $3-off coupon to reduce the cost of a box of Claritin, one of the 11 purchases I was trying to pay for with my flexible spending account's debit card. The coupon discombobulated the computer that runs the flexible spending account for Time Warner (TWX, Fortune 500), my employer. Rather than rejecting just the Claritin purchase, or rejecting just the coupon, the system kept denying the whole purchase, without saying why. Chris Otto, who owns the pharmacy, realized remarkably quickly that the coupon was probably the problem, and designed a work-around that allowed us to get the transaction done.
By contrast, by the time the few humans left at the NYSE figured out what the problem was, the Dow (INDU) was down more than 1,000 points. These plunging prices made absolutely no economic or market sense, but the computers kept trading back and forth with each other, and the meltdown fed on itself. That's because computers don't think, and don't care. They just do.
It's almost the same problem the market had on Oct. 19, 1987, when computer-driven "portfolio insurance" trading programs kept pushing stocks down, inflicting a 508-point, 22.6% loss on the Dow, for its biggest one-day percentage loss ever. Maybe we've learned a little bit since then, because yesterday's loss was only about 10% before humans stopped it. But it still creeps me out.