All this would be frustrating on its own, were it not for the infuriating letter that at least 300 taxpayers received from a mysterious manager within the IRS, which rejected their return entirely. It read: "Your return includes income or tax liability for more than one taxpayer, other than husband and wife." Signed by "J. Bell," a person whom the IRS doesn't know about or doesn't seem willing to divulge anything about, the letters would have to feel very much like a personal dig. In a statement this week, the I.R.S. said that the letters had been "incorrectly sent" because of a processing error and that it "apologizes for this mistake and sincerely regrets any inconvenience to taxpayers."