There was a stranger sleeping in the Lincoln Bedroom when a maid opened the door on her morning rounds at the White House, and within minutes the Secret Service was going crazy.

President and Mrs. Carter were not in residence and in no danger, so agents restrained themselves from approaching the figure beneath the coverlet with their guns drawn.

The usher's office, which keeps a log of everyone entering and leaving the private family quarters, could not identify the intruder. Miss Lillian was visisting, but she was alseep across the hall in the Queen's Bedroom.

Awakened by the whispered hubbub, the president's mother was furious when the mystery was finally solved. Someone had stuffed pillows between the sheets the night before, and that someone was "a jackass," in the opinion of Miss Lillian.

"You'd have done it yourself if you'd thought of it," she was told.

"You're right, I would have!!" shecrackled, and ended up promising never to tell Jimmy and Rosalynn anything about the incident or the pandemonium it had caused.

The Lincoln Bedroom caper was one event in the life of 24-year-old Lucie Langford, who just ended a two-month stay at the White House as a kind of semipermanent house guest. It was one of her friends who staged the prank. Langford is the sister of Judy Carter, wife of the president's son, Jack.

When President Carter introduced her to Ireland's Jack Lynch, he identified her fondly as kin who had "come here on Labor Day for a three-day visit and she's still here."

Langford, a University of Georgia graduate who used to work in New York doing public relations for designer Calvin Klein, had come to Washington job-hunting. She has just moved into her own place after going to work for the National Geographic, researching "anteaters and aardvarks" for a new dictionary of mammals, the first copy of which has been promised to Amy Carter.

"Are you STILL here?" the president would tease, as her brief visit lenthened into weeks and then months.

She lived in one of the third-floor bedrooms Rosalyn Carter recently had redone by a decorator from Americus, Ga. She had a perfectly wonderful time, learning quickly never to come down to breakfast in her bathrobe because "you never know who you'll run into."

Among those she "ran into" unexpectedly was Billy Carter, who was just passing through town two weeks ago on his way to New York.

On another occasion, when the Carters were out of town, she was showing some friends around the private quarters and ran into Vice President Mondale, who was also showing some friends around the private quarters.

Landford and her friends waited hopefully in the hallway for Mondale and his entourage to catch up with them and perhaps stop for introductions and a chat. But Mondale, perhaps feeling sheepish about appearing to be casing the joint when the current tenants were absent, shepherded his group off quickly in another direction.

Langford sometimes ate at a nearby MacDonald's, but usually she took her meals with the Carters or the servants "just put leftovers on a plate and left it in the warming drawer."

Once she and Amy tried baking a pecan pie and cookies in the family kitchen, Langford says, but Amy and a friend made buzz-bombs out of flour and water and got into the indoor equivalent of a snowball fight. The place was a disaster area, according to Langford, "with butter all over the floor," and a cleaning crew summoned to cope with the shambles the next morning were making noises about banning the girls from any future culinary undertakings.

Langford loved the White House parties, but she was disappointed when someone introduced her to the pope and then neglected to introduce her to Gregory Peck, who had been standing "just three feet away."

Chip Carter's "mystery" party last month at the White House -- the one with some 1,200 guests, which the White House is being so secretive about -- was almost canceled because of the Iranlian crisis, Langford says.

But with so many people invited, and the band and the food and logistics already arranged, the decision was made to go ahead.

Spokespersons on Mrs. Carter's staff have consistently rebuffed questions about who paid for the party and its purpose.

Another said the party was "official" and not private, although the press was barred. Did "official" mean it was paid for by the taxpayers?

"Undoubtedly!" was the reply, as she slammed the receiver down.

Whoever made up the guest list couldn't have culled through it very carefully.

One inquistive guest was unable even to find out from people who were there why they were invited. Almost everyone was under 35, she reported; most of them were lawyers and doctors and professional people, many of them from out of town.

The party, Chip Carter told her, was "something he wanted to do for his peers." But many of the guests, when questioned, said they had no idea how or why they had been invited.

Several former Carterites who have defected to Sen. Edward M. (Ted) Kennedy's campaign headquarters received those engraved White House invitations.

They declined, but at least one invitation went up on the Kennedy bulletin board.