When Major Payne takes Mrs Walbern out to eat he eats his food real fast. The camera switches to behind him looking over him to his date and you can see the food is still on his plate, even though he has already ate it.
No, it really is a slip-up. I noticed it too, and I've watched it over and over again to figure out if it's someone else's plate or if it's an extra plate or what. But it really is his plate that shouldn't have any food on it and does.