As Benjamin Martin is talking to various people after the meeting in Charleston you can see a boom mike appear at the top of the screen at least twice.
I don't think the boom mikes are visible on the home video/dvd versions. If you're talking about something you saw in the theater, then it is more a projection error than a slip-up. For example, when I was watching Jackie Brown in the theater, the projection room staff totally messed up and for a few scenes we were watching nothing but foreheads and boom mikes!