Corrected entry: Near the beginning of the film, Tony Stark views the information on the other superheroes before they all come together, on hologram touch screens. If you look closely at one of the screens, you can see footage of Black Widow and Hawkeye fighting side-by-side. That same footage was taken from the final fight scene of the movie, so it hadn't happened yet.
BocaDavie
7th May 2012
Correction: Not the same footage; the background is different, and they're on different sides of the screen. The footage they are showing is supposed to be of a previous battle the two of them had fought - given the broadly similar setup and Black Widow saying "just like Budapest all over again", it might be that occasion.
BocaDavie ★