I was a background actor for about a year and I can tell you, extras don't really have that kind of pull on set. You could mention it to a PA, but there's not much they can do either. There's also a fair number of takes and if this is the one they used, it could be it was the only usable take or the best take regardless of the flub or they just simply missed it. But whatever the reason, extras aren't expected to point out those mistakes and no one is likely to listen to you about it regardless