Many movies, no matter how much the whole thing may have sucked, have at least one scene that will make you uncontrollably and audibly say, "Oooohhh Ssshhiit!" Now this scene can come from any movie, but what's yours?
Here's probably one of the most famous to get it started:
This scene right HERE from FULL METAL JACKET.
It's the scene where joker finds Gomer in the bathroom with his gun. That whole scene from when Gomer looks up and says "Hiiiiiiiii Jooookeeeer" to when he blows his brains out I seriously was like "oh shit".
There's a scene in the middle of Troy where Hector's basically laying waste to the Greek's while Achilles is off being whiny. Then Achilles inexplicably comes out and starts fighting Hector one-on-one. Even though the guy's clearly moving just like Achilles and wearing Achilles' armor, we sense something amiss as he doesn't seem to be doing well at all. Then memories of The Iliad leapt back to me, and I was all like "Oh shit!" as the fight ended, and it's revealed that it was Achilles' young cousin, not actually Achilles. By coincidence or intention, the timing of that scene panned out perfectly for my first time watching it.