Apparently, they aren’t allowed to change their votes if it would alter the overall outcome. So even though she realized it right when she pressed the button, the override stands.
From the article on WRAL:
Carney characterized her vote as “very accidental.”
“It is late. Here we are rushing to make these kind of decisions this time of night,” she said.
Carney pointed out that she has voted against fracking in the past, and said she spent the day lobbying other Democrats to uphold the veto of Senate Bill 820.
“And then I push the green button,” she said.
Just after the vote, Carney’s voice could be heard on her microphone, saying “Oh my gosh. I pushed green.”