Matt Yglesias reminds everyone that gaffe-centric press coverage of elections is foolish, because gaffes don't matter very much.
I'll break it down a bit more:
In general elections, most people vote based on party, with a bit of economic circumstances and (the overlapping factor of) approval for the incumbent president thrown in. Everything else is relatively minor. At best, a gaffe might have a very short-term effect, so one that happens in the days before an election might make a very small difference. Anything earlier -- including during presidential debates -- won't.
In nomination politics, during the (current) invisible primary phase, what's mainly happening is that party leaders of various types are assessing whether the candidates will support their issue demands and whether the candidates are apt to win. Gaffes can matter indirectly to the second of those; a candidate who develops a reputation for being gaffe-prone might lose support from those party elites who are focused on winning.
During the primaries and caucuses, gaffes may well be important. Voters are seeking to differentiate between very similar candidates, and someone who says something stupid during the week before a primary could easily lose support that week (although it's unlikely to be a long-term effect; a gaffe now wouldn't hurt next February). However, remember that the primaries and caucuses phase of the nomination contest is heavily structured by what's happening before the voters get involved...George W. Bush and Al Gore had basically wrapped things up before Iowa in 2000, for example, so either could have easily withstood even a significant gaffe -- and it wouldn't have mattered what the other candidates said. And (via Nyhan) even when the circumstances seem ideal for a gaffe to matter, it's still rare for it to make much difference, as two John Sides posts about 2008 show.
So that's it. Basically, it's just not that big a deal.