I don't know where this actually fits into the whole mutation limitations. But currently i'm participating in a cross biology/chemistry internship. And one of the groups that is particupating is looking into DNA double strand breaks. From what little I understand about biology, and what has been described, is that double strand breaks, a single one is enough to kill a cell. At this point it is pretty much believed that double strand breaks occur at a rate of 10-60 times per cell...
So big question that arose out of that why aren't we dead?
And this is where DNA polymerases come in. These things act as a way to get double breaks to react with either the other end, or another double break. So basically dna strands recombining willy nilly all the time in our body (again to my bare understanding i don't know much about DNA). So 'mutation' rates in the sense that cell DNA in body is no longer initially the DNA that it started with is sounds like its a pretty frequent occurance no matter what way you look at it...