An overbite or underbite, severe or not, is never going to be a survival advantage.
I wouldn't be too sure of that. Hypothetically, if all else fails - which it doesn't, as lyx2no's pictures show - an overbite could be subject to sexual selection.
Usually, when you have to resort to an argument from incredulity, there's something you have overlooked.
"Ignorance more frequently begets confidence than does knowledge: it is those who know little, not those who know much, who so positively assert that this or that problem will never be solved by science." - Charles Darwin.