Iron Man (2008)
The Marvel Cinematic Universe is literally chock full of Easter Eggs. It’s kind of their thing to give these little gifts to their fans. They make reference to all of their movies within their movies, kind of like an inception of Easter Eggs. Iron Man was made in 2008 and was the first movie of the entire franchise.
There’s a very important scene in the movie that shows Tony Stark getting dressed in the iconic Iron Man suit. If you look closely, you can make out a very important object that will actually be showing up in most future movies. The item you’re seeing is actually Captain America’s shield, or rather, half of it. At this stage of the story, we’d guess it was probably halfway through production.