IT's always difficult, and a little embarrassing, to deal with an uninvited guest. The White House had given much thought to the prospect before the Inaugural. A small army of servicemen, plainclothes officers, uniformed police, and Secret Service agents was deployed to protect the premises. They sealed the manhole covers. They planted enough concrete barriers to stop tanks. They were armed with surface-to-air "Stinger" missiles. Officials were assigned to mingle warily in the crowds. Sophisticated "sniffer" devices were on hand to, well, sniff out explosives. There were bulletproof vests and bulletproof limousines. Mobile command buses were ready to handle any emergency. "We don't take anything for granted," said Joseph Yeldell, director of the D.C. Office of Emergency Preparedness.

Robert Latta, a water meter reader from Denver, wanted to join the party. There was a lot of excitement in town, and he just went with the flow. The Marine Corps Orchestra was filing into the White House, and Mr. Latta filed along with them. Once inside, he slipped away on a 15-minute self- guided tour of the state floor of the mansion and the family dining room.

But then he began to attract the attention of some of the security people. If he was supposed to be a Marine Corps musician, why wasn't he in a Marine Corps uniform? "That's what gave them the clue," Larry Speakes, the president's press secretary, subsequently explained. Mr. Latta was arrested shortly before the inauguration ceremony.

It could have been worse. At least he didn't wander into the Reagan family's quarters that morning the way Michael Fagan slipped past security guards three years ago and had a chat with Queen Elizabeth in her Buckingham Palace bedroom.

Everyone who had a proper invitation to the White House for the occasion will naturally have been outraged by Mr. Latta's visit. Everyone who did not have an invitation will have mixed feelings. It's always a little uncomfortable to be reminded of the fragility of even the most elaborate security precautions that surround the president. But it's also hard to suppress at least half a cheer for the free spirit who gets the best of the big organization with its tank traps and its sniffers. Does Mr. Latta rate more than half a cheer? No, alas -- for not every free spirit might have purposes quite so innocent as Mr. Latta's.