First and foremost, take it offline (don’t do it in front of anyone else). Second, do it in person. Third, as you approach your colleague or teammate, point out or question the facts of the mistake rather than directly blaming her. Don’t make it about “you did something wrong.” Instead, position it as: “I’m concerned there might be an error here” or “I’m wondering whether or not that was the intended outcome we were hoping for?” Or “It seems to me like there’s a mistake in the presentation that just went out to the client, do you have a few minutes to discuss?” Let the facts speak for themselves, and make it clear that your main interest is what’s best for the client/company, not in laying blame.