BY DAVE DAVIES/DAILY NEWS You can lose an election at the ballot box, and sometimes you can lose one with the slip of a pen. Mayoral candidate U.S. Rep. Bob Brady failed to list his city pension on the financial-interests statement he filed with his nominating petitions Tuesday, leaving him open to a challenge that could potentially end his campaign. “It may well be an error,” Brady campaign spokeswoman Kate Philips said late yesterday, “but it was not a purposeful omission. We plan to file an amendment.” But that may not be enough if another candidate challenges Brady’s candidacy in court. The statement of financial interests requires candidates to list sources of income, and several have been removed from the ballot in the past for omitting income sources, even when it seemed clear it was an innocent omission. In 2003, City Council candidate Vernon Anastasio failed to list his state legislative job, and was permanently stricken from the ballot. In 2005, former Common Pleas Judge John Braxton was removed from the ballot for city controller after an extended court battle over his failure to list several sources of income. A Commonwealth Court panel ruled that Braxton could amend his filing and get back on the ballot, but the state Supreme Court overturned that decision, declaring Braxton’s omissions were fatal to his candidacy.