And Then There Were None - S6-E16
Revealing mistake: Samuel is very much dead, laid out on the table in the canning factory employee lounge, but as Sam stands over him, talking about whether or not it was right to gank him, you can see Samuel's chest rising and falling as he breathes. (00:26:30)
Revealing mistake: As Dean digs into the dead guy's guts and discovers the claw the pale dead guy blinks.
Revealing mistake: When the woman is under the bed with her baby her arm is covered in blood, but when she touches the baby's face and clothes, no blood comes off onto the baby.
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer