I would say that incidents like these are fairly uncommon. But tend to happen regularly anyway. In the last nationwide election poorly thought out statements are attributed with costing several candidates the election.
Mitt Romney's infamous 47% statement is widely considered by conservative pundits as the quote that killed his campaign.
“ All right, there are 47 percent who are with him, who are dependent
upon government, who believe that they are victims, who believe that
government has a responsibility to care for them, who believe that
they are entitled to healthcare, to food, to housing, you name it.
That that’s an entitlement. And the government should give it to them.
And they will vote for this president no matter what.” source
Senator Claire McCaskill was reelected despite being caught "misstating congressional expenses" After her opponent stated that "Legitimate Rape" did not result in pregnancy.
Missouri Sen. Claire McCaskill (D) successfully defended her seat on
Tuesday against opponent Rep. Todd Akin (R), the Tea Party-backed
Republican who has become infamous for his comments about "legitimate
rape." Source
In 2010 after opening up a huge lead in a Senate Race Chistine O'Donnell's statement of "I am not a witch" is credited with turning the tide against her as voters started to consider if she was a serious candidate.
Both Romney and Akin stuck to their statement for several days until it became apparent even to them that the statements were going to damage their campaigns. But by the time they tried to reverse course the damage was done. Akin fell from a significant lead to be blown out in the election by nearly 10 points.