It is extrmely rare for a female to allow a male to mount her when not in heat, but it has occured at times. However, if truly not in heat, they cannot get pregnant at this time as they are not ovulating.
Once thing to consider which is more common than mating while not in heat is a silent heat. Some dogs at times (or some dogs all the time) have silent heats, in which their body is indeed in season and they are ovulating, but no discharge comes out. It is very difficult at times to determine this at home, as sometimes the vulva does not even swell. But a vaginal swab, a vaginoscopy or a bloodtest can be helpful in determining if the dog is indeed in heat at that time. Other than the possibility of getting pregnant when not wanted, silent heats are not a problem.