When a clerk in a department store makes a mistake, it is irritating but usually no big deal.

If you ask for $5 worth of high test gasoline and get a couple of extra squirts, no problem!

But let the government make a teenie weenie mistake and, wham, bam, shazzam, you can have an eye-popping federal case on your hands. Take the matter of the "mistaken" 80-day furloughs, or the $1,000 swords for the Marine Corps:

* Because of a congressional error, 923 employes of the Inspector General's Office at the Department of Health and Human Services face the possibility of being laid off for 80 days without pay. It probably won't happen, but it could. This is what happened:

Last December Congress, anxious to start its month-long Christmas vacation, approved stopgap funding bills that would keep various federal operations--HHS among them--going until the end of this month.

Congress failed to clear regular budgets (for the fiscal year that started last October) so it approved "continuing resolutions" right and left, to let agencies keep spending pending approval of their real budgets, which still have not been okayed.

The House voted $88.1 million for the IG's office at HHS. The Senate chopped $12.1 million from that. The lower figure became part of HHS' continuing resolution.

Problem is that the Senate, because of a glitch in its bill, made it appear that the $12.1 million cut was to come entirely out of the inspector general's payroll, even though IGs, who root out waste and fraud, are among President Reagan's favorite federal people.

Unless the error is corrected, by month's end the IG's office, including 191 people here, will have to begin furloughs for everybody. In the worst case, if nothing is done to resolve the problem, workers would be out for at least 80 days between now and the end of September.

HHS officials, Senate and House brass are confident they can correct the matter, via an amendment to the pending continuing resolution, in time. But if they do not, the error will cost nearly 900 people about 80 days' pay this year.

* Interesting goof number two concerned a contract that the government let for sabers (as in George Armstrong Custer) and scabbards for the Marine Corps. The official announcement said that the corps had purchased 147 swords for $142,975. That raised a number of interesting questions:

Was the Marine Corps going back to the Shores of Tripoli? And if so, wasn't $972.92 a bit much for a sword?

Or suppose penny-pinching Sen. William Proxmire heard about them, or Libyan Col. Muammar Qaddafi, who has no sense of humor, learned the corps was beefing up its armaments with $900 swords. For that matter, what if the U.S. Army heard about it?

But not to worry. It turns out the swordplay wasn't that serious. The Marine Corps Contracts Division, Logistics Base, in Albany, Ga., says somebody--a civilian, wouldn't you know it--in Washington, got the numbers fouled up. What the corps really ordered, a spokesman said, was 1,487 swords (for ceremonial purposes) at a total cost of $1,992. Who could argue with that?