Wet Nappy
After 3 years, the wife starts to think that their child looks different, so she decides to do a DNA test. She finds out that the child is actually from completely different parents.
Wife: "Dear, I have something very serious to tell you."
Husband: "What’s up?"
Wife: "According to DNA test results, this is not our child."
Husband: "Well don’t you remember? When we were leaving the hospital, we noticed that our baby had a wet nappy and you said, “Eddie, go and change the baby, I’ll wait for you here.”