A man phoned me on Saturday night and said, "I'd like to ask a question."

"Ask," I said.

"Why do manufacturers so often design one monstrous flaw into otherwise excellent products?" he inquired.

"I don't know," I said. "Why do you ask?"

"Oh, you know," he said, "like when they offer an appliance that really meets a consumer need -- except that it is designed in such a way that it may give the user an electrical shock.

"This afternoon I read an article in the new issue of Prevention magazine about how walking is one of the most beneficial activities we can engage in, so I decided to take a nice brisk walk before dinner.

"By the time I was ready to begin, ti was snowing and there was a powdery layer of snow on the ground, so I put on my beautiful new boots -- fleece-lined and warm as toast. They were just perfect for an invigorating stroll in the snow except for one tiny detail: the soles and heels consist of some kind of man-made material that would be ideal for the bottoms of skis but is wholly unsuitable for the bottoms of bad-weather boots because it is super-slippery. I fell twice and damn near fractured my dignity. Why do manufacturers do things like this?"

I don't know. Perhaps it's because the manufacturer, for his personal use, prefers a competitor's boots, so he jsut doesn't know how his own perform.

I love a local supermarket's chocolate cake but seldom buy it because there is no way to cut off a slice without utterly destroying the transparent material in which the cake is wrapped.

I have searched theses packages diligently for an end that can be peeled back intact and then resealed, but I have never found one. I always end up with a lot of cake crumbs and a need to repackage the remainder.

Why, Mr. Supermarket? Why? POSTSCRIPT

I think Prevention magazine carries some excellent health articles, but also a few that leave me wondering whether enough research has been done to warrant them. Prevention favors common sense and "natural" therapy (like that advocated in the article about walking). It carries many advertisements for vitamins, "health foods" and similar products. The magazine is published monthly by Rodale Press. It costs $1 and issue, $9 a year. Its mailing address is Prevention, Emmaus, Pa. 18049. 'TWAS EVER THUS

While driving to and from work on Saturday and Sunday, I felt the elation that comes with discovery. Perhaps Sir Isaac Newton experienced a similar emotion as he sat under an apple tree and discovered gravity.

Gold's Law now includes this additional observation: "The more slippery the roadway and the less certain that vehicles traveling upon it will be able to stop, the more likely it is that a pedestrian at the curb will suddenly dart out and try to cross an intersection, especially when he is faced with a red light or a 'Don't Walk' sign."

Please insert a clipping of this item in your Gold's Law loose-leaf notebook under the section headed, "Mortimer, How Can You Be So Stupid?" n

The most appropriate place to put it might be right behind the three single-spaced pages that deal with drivers who also live dangerously.

This is the section that beings with reference to the motorist who doesn't bother to clean the snow from his back window because he intends to drive forward, not backward. It ends with a discussion of the blithe spirit who doesn't realize there is snow on the ground and drives at his normal rate of speed, which is abnormally fast.

The only drawback to cleaning off your back window is that when you are stopped at a traffic light you can see the Barney Oldfield types bearing down upon you at terrifying speeds, and until they have actually come to a stop without crashing into your rear end you can do nothing except sit there and cringe as you await the sound of metal and glass being shattered.

Those who don't clear their back windows are mercifully oblivious to what's going on behind them. If ignorance is bliss, they may not be so dumb after all. CAMPAIGN '80

If Ronald Reagan hopes to match President Carter's upswing in the polls he's going to have to find some way to get Moscow to attack him, too. THERE'S A BIG DIFFERENCE

Bill Leary reminds politicians: "Free speech is a right, not a continuous obligation." THESE MODERN TIMES

Herm Albright, sage of the Perry Township (Ind.) Weekly, comments:

"In today's movies, there's more trash on the screen than on the floor."