2. Super-Soldier Serum (The Incredible Hulk)
You're no doubt aware that it was the mysteriously named "Super-Soldier Serum" that transformed the weak, skinny Steve Rogers into Captain America in
The First Avenger. But were you aware that this same formula was the one given to Tim Roth's bad guy Blonksy in
The Incredible Hulk, so that he would be able to battle the Hulk? Seriously: Blonsky is even told that the serum was developed "during WWII," and is granted super speed, agility and strength by way of a small dosage. Of course, he eventually succumbs to the serum's inherent flaws when it begins to decompose his skeleton and he transforms into the Abomination. This is another nice little touch that helps to connect the MCU, though, and a good way to introduce some of the plot points for a Captain America movie before that movie ever seemed possible. If you happened to be a Captain America fan at the time, this little easter egg would have likely had you wetting yourself in the aisles. Ew.