MSNBC pulled an abrupt about-face on Thursday, announcing that it had rehired a contributor just days after it fired him when far-right activists drew attention to an inflammatory tweet he posted in 2009.

“Sometimes you just get one wrong — and that’s what happened here,” Phil Griffin, the cable network’s president said in a statement on Thursday announcing that the cable network had rehired Sam Seder, a political commentator and host of the “Majority Report” podcast.

The episode began late last month when Mike Cernovich, the far-right activist, resurfaced a tweet sent by Mr. Seder in 2009 that was meant to mock supporters of the film director Roman Polanski.

Mr. Seder’s tweet, which has since been deleted, was posted shortly after Mr. Polanski, who pleaded guilty in 1977 to unlawful sexual intercourse with a 13-year-old, was arrested in Switzerland in 2009 in connection with that crime.