Nancy Ferris, secretary of a Northern Virginia church, was at work with pen, paper and the Internal Revenue Service's 1985 "Employer's Tax Guide" (Circular E) calculating withholding payments for church employes when she discovered a mistake.

According to the summary wage tables provided in the form, an employe earning $100 a week owed $7.50 in Social Security taxes. So did his employer. According to her calculations, using the new contribution level of 7.05 percent, the correct deduction should have been $7.05 a week.

But when she pointed out the 45-cent error to the IRS's public assistance personnel, she said, they gave no indication that taxpayers who followed the mistaken guide would be reimbursed.

Don't worry, IRS spokesman Larry Batdorf said when questioned yesterday. The Treasury does not want to keep the $23.40 overpayment that it would get from each employe and employer affected by the mistake and has taken steps to point out the mistake.

Employers who discover the problem can make adjustments in later quarterly tax returns, Batdorf said. But, he added, the overpayments would be so small on individual returns that "I don't know if we could pick them up" and inform an employer of his overpayment.

Notice of the mistake has been mailed to employers with their 1984 unemployment tax returns and has been posted in the December issue of IRS's monthly "Bulletin," which most tax accountants receive, he said. Separate tables elsewhere on the same page give the correct information.

"We did make a mistake, no question about it, and the best thing we can do is go back to every employer involved, which we did," he said.