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.
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.
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: The pool allowed Thor to relive the vision that Scarlett Witch made him see. It's through the vision that he finds out that Loki's staff contained an infinity stone and it would be to dangerous to destroy. He also sees the creation of Vision. He also sees that Asgard is in trouble. Since Selvig is an expert (as much as one can be) in Asgard and Earth he would be the one to help Thor find the pool. Many of the scenes relating to the pool were cut and could appear in the home release.