WASHINGTON  Timothy F. Geithner has been misidentified as a former Wall Street insider from Goldman Sachs so many times since he became the Treasury secretary that he and his advisers had taken to joking about it. Then the joke backfired.

Earlier this month, Mr. Geithner had breakfast in Manhattan with Mayor Michael R. Bloomberg and Robert Steel, a deputy mayor and former Treasury official in the Bush administration who had previously worked at Goldman. Facetiously, a Geithner aide said Mr. Steel and Mr. Geithner knew each other from the investment bank.

Later that day at a public event, the mayor in all seriousness referred to Mr. Steel and Mr. Geithner, and added, “They both worked at Goldman.”

Oops.

Just as the Geithner aide’s humor fell flat, likewise newspaper corrections, Mr. Geithner’s objections to TV news interviewers and his staff’s work to spread the boss’s résumé have failed to dispel the belief that Mr. Geithner is a former Wall Street banker, or more specifically, a Goldman guy.