PROMISES, PROMISES. Sometimes they backfire. A two-for-one coupon is a compelling reason for choosing a restaurant. But a restaurant's refusal to honor its two-for-one coupon is a frequent frustration of bargain-hunting diners.

Sometimes it is clearly the diners' fault -- they don't read the fine print, thus don't notice the offer is not good on weekends or holidays, or that specials are not included. However, sometimes the restaurant hasn't informed all of its staff what the rules are, and a promotion meant to draw new customers instead chases them away -- along with any friends to whom they pass on their complaint.

So it happened at the Hunan Gallery, when Trudy Summers and friends intended to use an Entertainment '88 card that promised two-for-one at lunch or dinner. When she presented the card after lunch it was refused because, as the staff person said, the restaurant was arguing with Entertainment '88.

When I called Hunan Gallery's manager, Mike Quach, for an explanation, he apologized profusely and explained that Entertainment '88 had made some mistakes on the card and had not specified that the deal did not apply to lunch or to chef's specials. The owner was, indeed, arguing with the company about that. But that was not the customer's fault, he said, and the customer should not have been told about the restaurant's problems or refused the discount. "The assistant manager tried to do his job -- explain to the customer the whole story," surmised Quach. "He should have honored it."

How can customers avoid facing such an embarrassment and disappointment? Entertainment '88 has a 24-hour hotline detailing which restaurants have closed and whether any are in litigation with the company over honoring the cards. (Hunan Gallery was not mentioned on the hotline announcement when I called five days after the incident.) If a customer has a problem the hotline hasn't warned about, he should call Entertainment '88, said a spokesperson; the company will usually offer free theater tickets or something as an apology.

To further avoid problems, suggested Quach, "When they come in, first show the Entertainment card." That not only assures ahead of time that the customer will receive his discount, it allows the management to keep track of how many customers the Entertainment '88 promotion brings.

While Entertainment '88 is a discount- coupon company that has been in business many years and sells its $30 discount books through such established nonprofit organizations as the American Cancer Society and the Jaycees, occasionally fly-by-night companies sell coupon books that don't have much reality behind their offers.

Before investing in a discount book, ask for the full list of restaurants to see whether they are ones you would be inclined to visit even at half price; while Entertainment '88 offers discounts at hundreds of restaurants, carryouts and public events, some discount books list fewer restaurants than their advertisements lead you to expect. Then call a few of the restaurants to check their commitment to honor the discounts and whether there are restrictions not mentioned in the brochure.

And if a restaurant refuses to offer the discount it has promised, try calling the coupon company right from the restaurant so the management has to explain itself to the company in front of you.

OOPS! -- The bad news is that we made a mistake on the price of Saturday night's Valentine Champagne Gala, sponsored by the National Kidney Foundation. The good news is that the event, at the old Iranian embassy, costs $100 per couple, not per person. For reservations call 337-6600.