Any science-fiction movie is predicated at least a little on the suspension of disbelief. But then there are points where what we're presented with so defies logic, physics, or natural human behavior that the mind rebels. At that point, the credibility of an entire movie can be at risk.
For example, early in the new, sci-fi/vampires-rule-the-world flick Daybreakers, Ethan Hawke enters the world headquarters of the international corporation that is the sole, global supplier of human blood. He steps out of an elevator and beholds before him a vast, seemingly limitless atrium, stacked row upon row with stainless steel racks containing human bodies being "milked" for their blood.
Think about that for a second. Think about where that harvest is taking place. Imagine going into McDonald's world headquarters and finding that, just as you step past reception, you're confronted with a fully operational slaughterhouse and hamburger processing plant. Does that make sense?
Even the best of movies can have its "Oh, come on!" moment (although, admittedly, those movies are far outnumbered by the really, really sucky ones). We take stock of our favorite jaw-droppers in Top 10 "Oh, Come On!" Moments in Sci-Fi Movies.