Most federal agency retirement counselors do a good job. But when they goof, it can be a nightmare. For instance:

Three years ago U.S. Customs Service worker Fleurette Seidman learned that she had cancer and asked about early retirement. She was officially advised that she was eligible for normal (nondisability) retirement if she would take a reduced pension. She agreed, signed the necessary papers and gave up her New York City apartment and moved to Florida in late 1984.

A few months later she was contacted by the government and told there had been a mistake: She was six years short of retirement eligibility. Seidman was told to return to work as soon as possible. She did in April 1985. The government made up her lost salary.

But it rejected her claim for $8,000 in expenses that she said were associated with her move due to erroneous retirement advice. After battling with the government, the National Treasury Employees Union, which represented her, took the case to Congress.

Tomorrow the House Judiciary Committee's subcommittee on administrative law and government relations will hold hearings on a number of private bills, including one introduced by Rep. Stephen Solarz (D-N.Y.) to give the $8,000 in claimed expenses to Seidman. If Congress approves the bill, and it probably will, Seidman's case will be virtually closed.

But for federal workers contemplating retirement there's a message: Check it, get it in writing and check again before walking out the door. If there was a break in service, or pension contributions were withdrawn or there is some other potential glitch, some bad advice may have been given. If there is a mistake, even if Uncle Sam makes and admits it, the problem is still the worker's.

Normal retirement for federal workers is at age 55 after 30 years of service; at age 60 after 20 years' service or at age 62 after five years' service. Pensions are based on salary and length of service. Workers in special categories, such as law enforcement, air traffic control and the Foreign Service, have different rules.Survivor Benefits

This week's Federal Times advised prospective retirees to be wary of advice that they buy life insurance or other forms of annuity rather than taking a survivor benefit for their spouse. Some private counselors say they can offer retirees a better survivor benefit than the one guaranteed by the government. The newspaper says such offers should be weighed carefully because the civil service benefit is hard to beat. Meetings

Social Security Administrator Dorcas R. Hardy will speak at Thursday's luncheon of the Association of Government Accountants meeting at the Baltimore Hilton Inn.

Jim Burnett, chairman of the National Transportation Safety Board, will speak at tomorrow's Federal Executive Institute Alumni Association luncheon at the Fort McNair Officers Club.

Virginia House of Delegates candidates will be on hand at today's 1:30 p.m. meeting of the National Association of Retired Federal Employees at the Lee Senior Center in Alexandria.

Montgomery County Executive Sidney Kramer will speak at the Sept. 22 evening meeting of NARFE's Wheaton-Glenmont Chapter at the Forest Glen Senior Center in Silver Spring. For information call 649-2672.