The San Francisco 49ers hadn't lost in two months. So when they carried that strange but seemingly sturdy 12-5 lead into the fourth quarter today against the Cleveland Browns, there was little reason to think things would change.

But they did.

First Brian Sipe, the Cleveland quarterback who had completed an un-Sipe-like eight of 23 for a meager 83 yards over the first three quarters, threw a 21-yard touchdown pass to Reggie Rucker with 6:46 to play. The score was tied and so were the vocal chords of the 52,445 in soggy Candlestick Park.

One failed 49er possession later, Sipe carefully, confidently drove his offense 59 yards in 3 1/2 minutes. Then former 49er Matt Bahr kicked a 24-yard field goal with 46 seconds left and the Browns defeated San Francisco, 15-12.

"When you win seven in a row like we just had," said 49er wide receiver Dwight Clark, "you figure that even if they took the lead, then something would happen and we would win."

An 8-2 record and a three-game lead in the NFC West can make a team think such precocious thoughts. But Joe Montana's desperation pass on fourth and five from his 37 was intercepted by Cleveland's Judson Flint with l0 seconds left. The joy ride sans defeat is over. San Francisco is 8-3.

"This is a sobering reality for us to lose," said San Francisco Coach Bill Walsh. "We should have and could have won it."

"Key mistakes . . . we had chances to win it," said Montana, the NFC's top-rated quarterback, who completed 24 of 42 for 213 yards. He didn't have a touchdown pass and threw two interceptions.

Fred Dean, the 49ers' defensive end and certified sacker, suffered a bruised sternum on a hard helmet hit early in the fourth period and didn't play the final 12 minutes. Sipe completed eight of l0 for 97 yards during that time.

These two events were slightly related.

"When Fred Dean went out," said Cleveland running back Mike Pruitt, who gained 76 yards on 18 carries, "we had our opportunity."

Cleveland Coach Sam Rutigliano, a man prone to humor and humility, said of Dean's departure, "I'm sorry about the injury, but not sorry that he was out of the game. I took a Rolaids and I was fine."

Dean is not fine. Even though preliminary X-rays showed no fracture, Dean could not even put on his shoes afterward. He couldn't bend over. "I'm very sore. Yes it was difficult watching at the end," he said, between winces and grimaces.

It had rained here all week and it continued throughout the first quarter. There were 8,507 no-shows and the grass area between the 30- and 50-yard lines on the south end of the field was in poor condition.

"I felt as if I was running in thick sand," said Clark, who caught six passes to add to his previous 56.

Cleveland cornerback Ron Bolton said, "We're used to bad conditions on our home field, so this field didn't bother us too much."

For three quarters, however, the 49ers were enough of a bother. Cleveland did score first when Marshall Harris dropped Montana for a safety eight minutes into the game.

"He just fell down and I fell on top of him," said Harris about the first first-quarter points scored against the 49ers in seven weeks.

San Francisco's Ray Wersching's two field goals (29 and 28 yards) were countered by Bahr's 28-yarder in the second quarter. So the 49ers led, 6-5, at the half.

Two more Wersching field goals (both 28-yarders) in the third quarter made it 12-5. The 49ers could have had more than three points on that second possession, but running back Walt Easley was called for a personal foul after catching a first-down, six-yard pass, and running out of bounds at the Cleveland two.

That penalty moved the 49ers out to the 17, where Montana yielded to Wersching three plays later. "That affected the outcome of the game. I'll learn from my mistake," said Easley, a rookie.

Sipe finished 16 of 33 for 180 yards. He threw one interception and he threw the pass to Rucker for the only touchdown of the game.