Continuity mistake: Near The End of the episode when Dean is being chased by the possessed truck, Sam tells him to take a left. After he's made the left, Sam tells him to go exactly 7/10ths of a mile and stop. Dean looks at his gage and it reads 70098.3. Dean looks up, then back at it: it changes to 70098.5, but the next time he looks down it says 70100.6 and when he stops it says 70101.0, meaning he's gone 2 miles too far. (Probably meaning it took two shots to get the scene right but they messed up on the gage.) (00:35:00)
Revealing mistake: During the love scene between Dean and Cassie, you can see the tape used on Cassie to keep the show PG-13. (00:23:15)
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer