It's the case of the missing mail, and it has baffled Fairfax County officials for weeks.

Somewhere amid the 400,000 annual applications for automobile registration stickers that residents returned to the county this year, about 500 were misplaced.

Fairfax officials concede they they don't even know if they received the applications or know for sure how many were lost.

"I find it very embarrassing," says Springfield Supervisor Marie B. Travesky. "It's the only time I can remember where the county has fouled up so completely. The county staff can't even tell us what happened so we can explain it to the people."

Because the complaints have numbered in the hundreds, the county has acknowledged there was a mix-up and has twice extended the deadline for sticker purchases. The latest deadline--tomorrow--will not be extended, officials said.

"We don't really know where the foul-up was," said James Sandifer, assistant director of the county's finance office. "We don't know whether it was with the post office or the bank or where."

The original April 15th deadline for displaying auto stickers was delayed after hundreds of residents complained that although they had made their payments--typically $20 a car--they had not received their stickers.

Various rumors have floated around the county government about what happened--the most prevalent being that a mailbag was lost or that some checks were cashed without being recorded by the county finance office.

While the foul-up involved less than 1 percent of the auto sticker applications filed, it was extremely annoying for many of those caught in the snafu.

Russell Latta, a 16-year resident of Springfield, said he mailed his check to the county in mid-March. When a few weeks had passed and he had not received his sticker, he called the county.

"They knew they had a problem," said Latta. "They told me to fill out another application and pay another fee." Latta said it took him about a hour to go through the process. "They're wasting the county residents' time."

Latta said he has not received his latest bank statement and thus does not know if his first check was cashed by the county. County officials said it will refund any extra payments.

The fine for failure to display an auto sticker is $25 if the driver has a receipt to show that one was purchased but not displayed, and $43 if one was never purchased.

County officials said this week the complaints have stopped and the lines to purchase the stickers are gone. The last batch of auto stickers went out a week ago Friday.