It was the usual midday, midweek madness at the checkout counter of a Rosslyn restaurant. But Patricia Adams finally reached the head of the line, and handed the cashier a $20 bill and a $10 bill to cover her luncheon tab of $22 and change.

Back came two ones and some coins.

"Wait a minute," said Patricia, "I gave you a 20 and a 10."

"No, you didn't," said the cashier. "You gave me a 20 and a 5."

Patricia repeated herself. So did the cashier. So did they both once more. Finally, the manager was called.

He snippily informed Patricia that this sort of dispute happens all the time, and it's usually started by customers who are trying to gouge the restaurant. He refused to give Patricia her five bucks, and when she asked whether the customer isn't always right, he delivered this parting shot: "I value my employes over my customers."

But that morsel of diplomacy only made Patricia madder. She got in touch with the owners, who gave her the $5 later the same day -- but not before she had to wait for four hours. It was, to say the least, a thoroughly unpleasant experience.

But it's also an extremely common experience. So I figured some Washington area consumer protection experts might have some advice on how to protect yourself from being treated the way Patricia was.

Judy Doctor, manager of consumer education, Montgomery County consumer affairs office, had this to say: "State when you hand the cash to the waiter or waitress what you are giving them. Say, 'Here is a 20 and a 10,' for instance. Or you could note on the bill, after adding it up yourself, as you hopefully always do, that you have paid with a 20 and a 10. Actually write it on the bill so there's a record."

Donna Crocker, director of the Prince George's County Consumer Protection Commission: "Be careful. To remedy the situation, you could have the drawer and the receipts counted and compared."

Jim Brown, public relations officer for the Better Business Bureau: "I understand it's against the law to deface money. But I'd say take a pen and in the corner of your larger bills, ink in your initials. Then you could say to the waiter or the clerk, 'Examine your tens and you'll find my initials.' "

Mary Carroll of the Arlington County office of consumer affairs: "I would suggest that you remain calm and don't become hysterical, or people won't want to assist you. A lot depends on your reaction. I've heard managers say, 'Well, I've got feelings, too.' "

My own advice would be to pay with a credit card or check whenever possible. And if you're sure an attempted ripoff has taken place, rather than a simple mistake, I'd recommend the comeback line that has served me well for years.

As the clerk hands you the disputed extra money, look him right in the eye, smile and say: "Thanks. And by the way, nice try."

I guarantee you the clerk will be a little less likely to "forget" the next guy's extra five.

Two items about a local resident named Ronald Reagan, the first about money, the second about protocol.

Money: Donna Marie Honsinger of Annandale has some friends in California who treat her the way out-of-towners seem to treat all Washingtonians. They assume she knows everything about everything. So they called up Donna one day and asked who signs the president's paycheck, how frequently he is paid and whether his check is deposited directly into a bank account.

Donna said she'd check with highly placed sources around town. Immediately, in a panic and without a clue, she called me.

I didn't know the answers, either. But now I do -- and now Donna can call her friends in California and keep the "everything about everything" myth alive.

"The president's paycheck is handled like any other federal employe's paycheck," said Art Siddon, a Treasury Department spokesman. It is signed by the director of the financial center that issues the check -- in Mr. Reagan's case, the center in Washington. As to frequency, "again, his check is handled like any other federal employe's," Art said. "Federal employes are paid twice a month."

Direct deposit? Art says that's a personal matter, but "just between you and me," he imagines the president takes advantage of this option. Art said he can't imagine the check being delivered to the White House every two weeks. Nor can I imagine the president standing in one of those endless lunch-hour lines at a bank branch along 17th Street.

Protocol: Richard J. Quance of Alexandria sent along a photo that appeared in The Post. It shows a hatless Mr. Reagan saluting the flag-draped casket of a U.S. Marine who was killed in Lebanon.

Dick says the president shouldn't have saluted. "Granted, he's the Commander in Chief of the Armed Forces, but he's still a civilian," Dick argues. "Civilians don't salute -- especially without a hat."

I was pretty sure I knew the answer to this one, and Army Major Greg Rixon in the Pentagon public affairs office confirmed my suspicions.

Technically, Maj. Rixon said, the president was returning the salute of the Marines in the funeral cortege -- not saluting the dead Marine. "It is correct for the president to return a salute because he is acting in his duty as Commander in Chief of the Armed Forces," Maj. Rixon said. In fact, the president should always return a salute, and should always do so whenever any member of the military salutes him. His hatlessness makes no difference. CHILDREN'S HOSPITAL

Just three more days to go in our annual fund-raising campaign on behalf of the hospital that keeps and makes kids whole. These groups have made late-in-the-game contributions:

The 18th and Columbia Road Business Association ($725 from the association's treasury, and from these member businesses: Kalorama Design Associates, T.V. Technicians Inc., Cosmo Liquor Store, Saxitone Tape Sales, Churreria Madrid Restaurant, Dravillas Real Estate Co., Avignone Freres, Alan Lock and Key Service, T's Grocery and B & W Garage).

The Belvoir Officers' Wives' Club Bowling League ($78).

The Apron Strings Club of Northwest ($1,000).

Employes of AT&T Communications, Customer Service and Billing Data Services, Fairfax ($800).

The staff of Group 260 (Measuring, Testing and Lamp/Discharge), U.S. Patent and Trademark Office ($145).

Employes of the Air Line Pilots Association ($526).

The New Carrollton Swim Team ($250).

The Hazardous Site Control Division, Office of Solid Waste and Emergency Response, U.S. Environmental Protection Agency ($240).

The staff of Safeway store No. 943, Capitol Heights ($29).

The Congressional Record Index Office ($115).

The National Honor Society at Robinson Secondary School in Fairfax ($50).

The men and women of the Arlington Jaycees ($100).

Employes of the Washington Financial Center, Department of the Treasury ($216).

And the china and silver department associates at Woodward and Lothrop, Seven Corners ($80).

A fine performance! Any other last-minute performers? Here's what you need to know: TO CONTRIBUTE TO THE CAMPAIGN:

Make a check or money order payable to Children's Hospital and mail it to Bob Levey, The Washington Post, Washington, D.C., 20071. The campaign ends on Jan. 24.