anybody experienced something like this on d700?

Hi, I wanted to ask the d700 owners if anybody had any problem like mine. Some days before shooting on vacancies with my son, one picture was totally underexposed and then ok. This happened other tree times the days after. I know from were the problem comes, but I have the problem that the body and the lens that made the problem are in the limit of the warranty. is it something to worry or something that happens rarely.
Thanks, Armand