Continuity mistake: During the party, Natasha has just walked away after flirting with Banner. On the bar behind him is a glass of red cocktail next to a silver cocktail shaker. In all subsequent shots while Captain America is talking to Banner, the two have moved much closer together. (00:27:00)
Continuity mistake: SPOILER - The wounds on Pietro's body change. The most noticeable one is on his right shoulder. The amount of blood decreases too.
Revealing mistake: When Captain America is fighting Ultron in the truck, for a brief moment you can see a safety line holding actor Chris Evans as he's hanging on the side of the cab of the truck.
Factual error: After the Scarlet Witch forces Banner to become Hulk, he goes berserk in Johannesburg, South Africa. A few minutes earlier they were near the ocean at a ship scrap yard. Johannesburg is many kilometres away from any ocean.
Continuity mistake: When the city takes off it is surrounded by land. However while it is flying it is suddenly over a large body of water, but when it crashes back to the ground it is over land again.
Continuity mistake: During the big chase on the streets of Seoul, Capt. America's shield starts out with a few scratches and smudges on it. Towards the end, these disappear.
Revealing mistake: At the very last (pre-credits) scene of the movie, as all the avengers are assembling, Scarlet Witch flies in, but it's not Elizabeth Olsen but a stunt double.
Visible crew/equipment: At the end of the movie, when Captain America and Black Widow are walking towards the camera to train the new Avengers, the reflection of the cameraman's feet are seen on the glass railings in the back as they walk backwards shooting the actors.
Continuity mistake: When Scarlet Witch enters the fight, after Hawkeye's pep talk, there's a payphone booth to her right, several feet away. In shots of her fighting, the double doors are seen behind her. When Hawkeye nods at her, in the next shot of her, she is standing with the phone booth behind her. And then she's not near the phone booth when Hawkeye and Scarlet Witch start to leave.
Chosen answer: This is a fairly US-centric gag. In Archie comics, the titular Archie has two competing love interests: Betty and Veronica. Given the Hulk/Banner's history with Betty Ross being able to calm him down, Tony Stark has named the containment system/Hulkbuster armor "Veronica".