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.
Trivia: Stan Lee makes his customary appearance whilst the Avengers and a load of War Veterans are enjoying a get together. Thor offers a 1,000 year old Asgardian drink to Captain America, and Stan Lee asks for some. (00:25:15)
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)
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.