It was a full-page ad, but a half-fare misunderstanding.

Today's New York Times carried a big, back-page announcement from Trans World Airlines offering its passengers discount coupons similar to those United Airlines and American Airlines have been advertising for more than a week.

It appeared to be a major change in TWA marketing strategy. TWA had said repeatedly last week that it would not match the half-fare discount cooked up by United to lure passengers back after its prolonged strike and rapidly adopted by American.

It cost too much, TWA officials said. But TWA also said that it might change its mind if it appeared that too much business was shifting to United and American.

Then came this morning's ad, offering not only half-fare coupons good on round-trip flights between July 1 and Dec. 15 but on an added sweeteneer: a $25 bonus on land arrangements for TWA tours to Europe, Israel or the Middle East.

It must have come as a pleasant surprise to TWA passengers.

Unfortunately, the ad was not supposed to run.

For reasons that are not clear, the Times ran an advertisement that TWA had developed in case it decided to enter the half-fare derby. It was not the ad that was supposed to run.

A TWA spokeman said the airline is "embarrassed and not very happy. We've told all the travel agencies that the ad was run by mistake."

The TWA spokesman said the "Times has admitted they goofed, and we've demanded a full retribution."

A spokesman for the Times said, however, that just who goofed is in dispute. It was either the Times or Ogilvy & Mather, the advertising agency for TWA.

Ogilby & Mather merely would say, "At this point, we're not making any comment. We have nothing to add."

The TWA spokesman said it was not clear what liability the airline may have to passengers as a result of the ad. TWA lawyers and Times lawyers met today to discuss that issue, the spokesman said.