This happens on poker night. It's right after Ben suggests couples therapy to Andie outside. After she tells him that she loves him, but doesn't have to like him right then, she motions for a kiss and then starts to walk away. When he says thank you, she waves her left arm up and in the next shot it is her right arm that is waving....
To avoid duplicate slip-up submissions, make sure you check to see that your slip-up hasn't already been found by someone else. Because in this case there were already two of the same before you entered yours.