Anita Marshall, a Justice Department lawyer on her way to work, fed $5.50 into one of Metro's new computerized Farecard vending machines at the Rhode Island Avenue NE Metro station yesterday morning.

The electronic contraption swallowed her $5 bill, spit back 50 cents and flashed a red sign, warning, "Out of Service - See Attendant."

Other Washington commuters, like Marshall, had their first encounter with the $53 million Farecard system yesterday, and the upshot was a mixture of confusion, delight, some surprises, some puzzlement, occasional malfunctions and mistakes. Farecard's opening day, a Metro spokesman acknowledged, proved "mixed - obviously the people are learning their way."

Farecards are magnetically encoded, wallet-sized cards that subway riders will use to pay their fares instead of coins, transfers or tokens. Using Farecards will remain optional for subway passengers for the remainder of June. On July 1, they will become mandatory.

Because of anticipated confusion and possible delays on July 1 when Metro also plans to open a new segment of the rail transit system, officials are urging commuters to lean how to use the new Farecard gadgets now and to purchase Farecards before next month. Starting Wednesday, Farecards will also be sold at some 150 other locations in the Washington area, mainly banks and savings institutions. The new 17-station line will run from National Airport to a stop near the D.C. Armory and Robert F. Kennedy Memorial Stadium.

Like many passengers who were briefly bewildered by the Farecard system yesterday, Marshall took the temporary disappearance of her $5 nill in stride. "There are still some bugs that need to be ironed out," she remarked, as a technician unlocked the Forecard vending box and retrieved her money. The $5 bill had gotten wrinkled in the machine's mouth, bringing the electronic mechanism to a halt. The mishap caused a few minutes' delay.

Other subway riders discovered other quirks in the Farecard machinery.

A few wanted to put $10 bills in the vending boxes, but found out the machines will accept nothing larter than a $5 bill. Farecards worth up to $20 may be purchased, but to buy such a card from a vending machine would require four $5 bills, 20 $1 bills or a similar combination of bills in small denominations or coins. Other passengers learned that the Farecard machines also reject pennies.

Robert Deems, an Amtrak trainman, mistakenly bought a Farecard worth 70 cents, believing it would entitle him to a subway trip and a transfer to a bus to Arlington. The Farecard was valid only for the subway ride and, a Metro attendant later told him, he would have to pay separately for the bus transfer.

Al Borland, a freelance photographer, inadvertently ended up with a Farecard worth $2, though he planned to take only two 40-cent subway rides. "I didn't realize I could get change (from the Farecard machine) until it was too late," he said. "It's like a miniature Las Vegas."

Susan Holley, a college student from Cincinnati on a family visit here, got caught temporarily inside Metro's Union Station stop because of a malfunction in Farecard equipment. She had paid a 40-cent fare, as required at mdday, for her card at another subway stop, but Metro's new electronic exit at Union Station would not accept her Farecard and let her out. "Technology catches me," he said philosophically. An attendant eventually let her out.

Despite such mixups, Metro officials and many subway riders themselves expressed delight with yesterday's venture into fare-collecting technology.

"It works better than our 24-hour banking machine," Fred Entin, a head bank teller, remarked after buying a Farecard worth $2 at the Dupont Circle Metro stop. Metro officials said they were pleased that many riders had tried out the Farecard machines and that malfunctions occurred only sporadically.