3. Angry Guy In A Little Coat (The Avengers)
I know Bruce Banner is the Marvel Cinematic Universe's foremost expert on gamma radiation, and they claimed they needed one of those to track down the Tesseract, but for the love of God, there wasn't some other way to track it down? A cube of unlimited power and the only thing it throws off that's detectable is gamma radiation--conveniently the type of radiation that created the Hulk and draws him into the story? But fine. I'll let that slide. Bruce Banner carries a lot of baggage with him€”roughly half a ton's worth in the form of the Hulk. Which they promptly put on a flying helicarrier. This, my friends, is the mathematical equation for disaster in the best of circumstances, let alone when you're bringing the God of Mischief on board. When the inevitable Hulk-out occurs, he almost kills the Black Widow, smashing through several important looking internal mechanisms trying to get to her which surely had to foul up something important in the helicarrier. Thor intervenes and the two promptly begin smashing their way through the ship in a knock-down fight.
It's one of those things that's never shown unless they can use it for dramatic storytelling purposes (COUGH, COULSON, COUGH), but it's hard to believe that no innocent bystander is ever seriously injured or killed during these kind of fights. Thor gets thrown through multiple walls and no one's in the way? Or that in itself doesn't destroy some kind of vital system that, I don't know, is essential to keeping that big ship in the air and fully functional? Hulk's presence also distracts several SHIELD agents from the main attack by the compromised Hawkeye and his crew, which perhaps leads to the deaths of SHIELD agents due to lack of back-up.