When said in a disbelieving, shocked, or surprised voice it signals you didn't see something coming. Its said when all the evidence shows you one thing, but you still are in disbelief.
Dude1: Dude did you see Ashley?
Dude2: Not recently why?
Dude1: *points to Ashley making out with another girl*
Dude2: No... *is said in surprised and shocked voice*
Dude1: Awesome right.
Dude2: Hell yeah!