It had been so long since the San Francisco 49ers lost a football game some no longer knew the sensation. Today, memories were refreshed.

The Los Angeles Raiders, playing well enough often enough, upset the 49ers, 23-17, and undoubtedly upset the majority of the 59,748 fans who comprised the third-largest crowd in Candlestick Park history.

Champions of Super Bowl XVI last January, the 49ers gained a 13-3 regular season record and a considerable amount of respect in 1981 by making the proper play at the proper time. This process was unintentionally reversed in the '82 season opener. Everything that could go wrong did go wrong, at least for the 49ers.

San Francisco lost yardage on penalties and five Raider sacks on Joe Montana and lost the ball on three fumbles and one interception. Thus did it lose the game.

"I forgot how it is to lose," sighed 49ers Coach Bill Walsh, whose team closed last year with eight straight victories. "It's terrible. No wonder I keep talking about retiring."

For the Raiders, who shifted their base of operation to Southern California this season after 21 years in Oakland, the victory indicated they may have returned to the dominance of two seasons past, when they won the Super Bowl.

Jim Plunkett's three-yard scoring pass to tight end Todd Christensen with 9:14 left in the game erased a 17-13 San Francisco lead and put the Raiders ahead to stay. Chris Bahr kicked his third field goal, a 43-yarder, four minutes later after Montana lost a fumble.

There was more good news for the Raiders. In Marcus Allen, the 1981 Heisman Trophy winner, the Raiders have a fine all0purpose back. Allen, a rookie from USC, was one of two publicized rookies in this game.

Allen made the yards -- 116 rushing, 64 receiving. The other rookie, Renaldo (Skeets) Nehemiah of the 49ers, conversely, made a big mistake.

A year ago, Nehemiah, the Maryland grad, was known only as the world's best high hurdler. He had not played football since high school. In what has come to be known as "The Great Experiment," Walsh gave Nehemiah a large contract and extensive coaching, trying to make him a pro wide receiver.

Twice today, Nehemiah was thrown the ball, and twice he caught it, once for 12 yards and once for 14. But it was the pass he didn't catch that proved critical.

On what was to be the 49ers' final drive, the sort that last year ended in triumph, Nehemiah ran the wrong pattern. Montana couldn't find anyone open, and forced the ball downfield. The resulting interception with 2:17 left by Oakland's Lester Hayes finished the 49ers.

"The designated route was over the middle," said Nehemiah, "but I ran deep down the left side."

He dragged the defensive backs deep, and Montana was stymied.

"This was the first game," said Nehemiah. "The guys were a little excited and anxious. We did a lot of little things wrong that will be corrected. I knew this game counted, and I couldn't afford to make errors. I had to catch the ball."

Said the other rookie, Marcus Allen: "I've got to learn to disguise some things better and learn to turn the sweeps faster, but I'm happy we won."

The 49ers, with a remade offensive line because of injuries, couldn't stop Oakland's defense from penetrating and couldn't help what seems to be a group of inadequate running backs.

The 49ers gained only 60 yards net rushing. "Famour" Amos Lawrence of North Carolina, whom Walsh had hoped would help this season, carried only twice for six yards and fumbled on a kickoff.