When Everett Arnold of Bethesda received a note last week from the Mercantile-Safe Deposit & Trust Co. asking for the return of $302.10, he says he nearly "hit the ceiling."

It turned out that the "Merc," an old-line Baltimore banking firm, had overpaid the interest to Arnold and other holders of $33 million worth of bonds issued in 1986 by the Montgomery County Housing Opportunities Commission. Now it wants its money back. Pronto.

"They didn't even say they're sorry," said Arnold, 70, a retired federal worker. "They just said give us the money back."

Arnold said the overpayment appeared to amount to about one-quarter of the interest sent him by the bank on Jan. 1.

At the Merc, which is trustee for the Housing Opportunities Commission bonds and is responsible for passing on interest payments to bondholders, embarrassed officials in the company's corporate trust department were tight-lipped about the snafu, refusing to say how it happened and how many bondholders were affected.

"There was an overpayment, and we've taken corrective action," said a spokesman, who refused to give his name. "And the actions have been effective."

He declined further comment, saying, "We're not at liberty to openly discuss the issues involved with the bonds."

Arnold ventured a guess of his own for the reasons behind the overpayment: because the bonds were issued in November 1986, the first interest payment in July 1987 was for eight months -- the last two months of '86 and the first half of 1987.

When it came time for the second round of payments this January, he speculated, Mercantile accidentally sent out interest for eight months again, rather than the customary six-month payment.

"These guys say send your money back, but they won't tell you why," Arnold said. "It's a terrible attitude."