Amy Davidson lists seven of them:
3. Where was Warren Buffet’s secretary? Where were Romney’s tax returns? Tax fairness came up, but they didn’t, and Obama did not really control that line of argument. Nor did he draw a connection between Romney’s secret budget plan and his unseen tax returns. As it was, Obama was strongest on the bad math. But Romney got away with a tautological rebuttal—"So there’s no economist can say Mitt Romney’s tax plan adds five trillion if I say I will not add to the deficit with my tax plan"—that, when you parse it, just doesn’t make sense.