It happens rarely, but it happens. It usually happens when I get distracted, disrupted or just something interfering with my regular pattern of zipping up. Otherwise, muscle memory just takes over, and I don't even remember doing it.
Personally, I don't think I could trust someone who have never had his fly open in public. Either their claim is a lie, or if they are actually telling the truth, it'd be like someone without any wrinkles in their shirt - I'd just be weirded out. Everyone could use a little trouser Sprezzatura now and then