CLAYTON • St. Louis County’s Department of Revenue has made a $2 million mistake, neglecting to add a surcharge on the tax bills of about 200,000 property owners.

The oversight means those owners are saving a few bucks when paying property taxes in the weeks before Monday’s deadline. But it also means that the Metropolitan St. Louis Sewer District is being shorted more than $2 million in 2012 tax revenue.

The blunder comes on the heels of another embarrassment for the department and the administration of County Executive Charlie Dooley. The county’s Collector of Revenue recently resigned after the Post-Dispatch reported that she had not paid her own personal property taxes since 2008.

MSD officials were not aware of the assessment mistake when contacted by the Post-Dispatch last week. On Thursday, county Revenue Director Eugene Leung cited retirements, attrition and general office turnover as the chief reason for the lapse.

Taxpayers who saved a few dollars, though, are not off the hook.

The county intends to recoup the fees — which come to approximately $5.30 for land or homes assessed at $150,000 — in a surcharge attached to 2013 property tax bills.

About half of the 400,000 properties on the county tax rolls are affected, said Garry Earls, chief operating officer for the county. Most of the properties fall in watersheds within the Interstate 270 loop. The MSD assessment is to help control storm water runoff.

“It was our error and we’ll correct it,” Earls said.

MSD spokesman Lance LeComb called the oversight an “honest mistake” that will initially cut into the $25 million in tax revenue planned for the current budget.

“Obviously, $2 million is a significant amount of money,” said LeComb. “But it should have a minimum impact as long as” the money is recouped.

County officials said the mistake was first spotted on Dec. 21. MSD was not notified until Friday because of Christmas vacation schedules, officials said.

Leung said the assessment mistake slipped through the cracks because of replacement staff in his department who were unfamiliar with the details of the formula that sets water and sewer tax obligations in multiple county jurisdictions.

“The language in the ordinance is not clear,” he said.

Earls defended the department, noting that complex tax assessments for schools, safety forces and other services swell the county tax book to over 1,100 pages.

“It’s disappointing to make a mistake like this, but it’s not unexpected,” he said.

Revenue Department employees neglected to add the surcharge, known as the “MSD extension,” to the main MSD assessment paid by property owners within I-270. The interstate loop loosely follows the boundaries of watersheds in a long-standing agreement between MSD and St. Louis County.

Properties inside that loop should have been taxed at a combined rate of 0.0821 cents per $100 valuation, but were only assessed at 0.0635 cents per $100.

Earls said state law permits the county to recoup the lost taxes without penalty.

Unfortunately, he added, the state’s leniency cannot spare the county “the embarrassment at getting our tax bills wrong.”

This month, the Post-Dispatch reported that Stacy Bailey, newly hired to oversee the county’s real estate and personal property tax collections, and her husband owed $4,801 in back taxes on their three vehicles. They had also filed for bankruptcy.

Leung and other county officials, while acknowledging they did not properly vet Bailey, initially defended her hiring and said she was the most qualified applicant for the job. But, after a firestorm of criticism, Bailey resigned.