Born Under a Bad Sign - S2-E14
Revealing mistake: When Dean turns over the deceased man lying on the floor, the man blinks while Sam and Dean are arguing about whether or not Sam is responsible for his death. (00:10:25)
All Hell Breaks Loose: Part 2 - S2-E22
Revealing mistake: Sam is breathing noticeably at the beginning of the show on the bed even though he is supposed to be dead.
Children Shouldn't Play with Dead Things - S2-E4
Revealing mistake: When Dean is talking to the girl about Angie, she's crying hysterically and using tissues yet there's no tears coming from her eyes.
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer