The Big Switch
After 10 years, the wife starts to think their kid looks kind of strange. So she decides to do a DNA test. She finds out that the kid is actually from completely different parents.
Wife: "Honey, I have something very serious to tell you."
Husband: "What’s up?"
Wife: "According to the DNA test results, this is not our kid."
Husband: "Well you don’t remember, do you? When we were leaving the hospital, we noticed that our baby had pooped. You said: Please go change the baby, I’ll wait for you here."