That reminds me of the old joke. Jesus comes across a crowd of people about to stone an adulteress to death. He orders the crowd to stop and reminds them, "Whoever among you is without sin may cast the first stone!" A little old lady limps forward, picks up a giant rock, throws it at the adulteress and instantly kills her.
Shaking his head, Jesus says, "Aw, Mom!"
Maybe I'm reading too much into what is just a joke but is it a Catholic belief that Mary was sinless?
