In an oversight that one city councillor described as a "colossal mistake," Cambridge overpaid its mayor by more than $30,000 during the past three years.

Mayor Kenneth E. Reeves '72 admitted Monday that he has been paid a $13,000 annual stipend for serving on the Cambridge school committee, even though the mayor's yearly salary--now more than $44,000--had been adjusted in 1988 to include that stipend.

In other words, he got the stipend twice.

The mayor immediately promised Monday that he would return the funds to the city.

Reeves said he only became aware of the overcompensation last Friday, when questioned by the Cambridge Chronide.

"This has just come to my attention, and I am very sorry to learn of it," he told the city council on Monday.

"I never once saw one of those checks; they were deposited directly to my account," the mayor said.

While city officials said the oversight was an honest mistake, the fact that it lasted six years has raised serious questions about Cambridge's massive bureaucracy.

"It is regrettable, and it is certainly disappointing, that a situation like this could go on so long," Councillor Francis H. Duehay'55 said last night.

Since 1982, the seven members of the school committee--including the mayor--have each received a stipend, now $13,000 per year, for their work.

Six years later, in May 1988, the city added $10,000 to the mayor's annual salary as compensation for the school committee seat.

But the mayor's stipend was never deleted from the school committee budget, and the committee said it was never told of the 1988 ordinance.

"We were doing what we believed was normal practice," said James R. Ball, the public information director of the school department.

"We had never been notified that there was any change in the arrangement we'd been doing since 1982," Ball said. "This never came to anybody's attention before last Friday."

A copy of the school committee's fiscal 1994-95 budget obtained by The Crimson still includes a salary item for $91,000--or $13,000 for each of the committee's seven members, including the mayor.

Ball said the oversight was not detected prior to last week because Reeves' predecessors, Alfred E. Vellucci and Alice K. Wolf, had turned down the stipend, and the money had been returned to the committee.