Invasion of the Body Snatchers/Headscratchers


 * In the 1956 film, how does the conversion process work? The dopplegangers are grown in pods, syphoning their target's memories until the mark falls asleep. Something unspecified happens to the true person's body, then the Pod Person takes their place. However, around the end of the film Original Becky falls asleep for the tiniest moment, then wakes up as Pod Person Becky, no pods or switcheroo required!
 * Typical 1950s B-Movie science fiction logic. It's the same reason why ants can grow to be 50-100 feet long (Despite that being biologically and physically impossible), aliens are able to survive on earth (even though there's a good chance our atmospher, gravity, and whatnot would be fatal to many alien lifeforms), and whatnot. Plus, "suspense overshadows logic". In other words, in order to build tension, movies have a habit of throwing aside established "rules" of the film.