Corrected entry: Old man hums "I love you", the Barney song, released in 1998. Movie takes place in 1977.
Corrected entry: The movie opens with the Clash's song "London Calling", released in December, 1979. The movie takes place two years before the song's release.
Correction: The song plays over the movie, not within the movie itself. It's no different than the movie's score being played over certain scenes despite being written in 2015 or 2016.
A filmmaker often uses music to reflect not only the decade but also the year the movie is set. By your reasoning, ANY type of music could be played over the movie that didn't reflect the era or year, so Oasis, Megan Thee Stallion, Sia, etc., would be OK, even though none of the artists I listed are from 1977 and reflect that year. My movie mistake listing regarding The Clash's song is correct and accurate.
Often, not always. There are countless examples of a movie soundtrack featuring technically "anachronistic" songs but they're not explicitly in-universe, so don't even count as deliberate mistakes, it's just an artistic choice. TV Tropes has an entire section covering this and similar things: https://tvtropes.org/pmwiki/pmwiki.php/Main/AnachronisticSoundtrack. It comments there how A Knight's Tale uses versions of modern songs, and the director pointed out that an orchestral score wouldn't have been period-appropriate either. The music making up a movie's soundtrack doesn't explicitly have to be from or before the year of the movie's setting.
No, your listing is not correct and accurate. You said yourself that filmmakers often use music to reflect a time period, "often" being the keyword, as in not "always." The Hateful Eight has "Apple Blossom" by The White Stripes playing over a part of the film, and that song wasn't released until 130 years after the movie's 1870 setting. Since it plays over the film and not within the film, it is not a mistake.
Corrected entry: Near the end when the children levitate and the mom, sister and brothers all run outside in the rain, the mom tells the kids to go to Peggy's. She's Peggy.
Correction: If you watch the credits at the end of the movie the neighbor's name is Peggy Nottingham.
Corrected entry: Peggy Hodgson gets her children out of the house, save for possessed Janet, telling them to go to Peggy's house while she attempted to re-enter their own home to save Janet. (01:49:03)
Correction: I'm not understanding what the mistake is. Are you saying it's a mistake that she said "Go to Peggy's house" because she's named Peggy? Because that's not a mistake. She's referring to Peggy Nottingham's house, who is a different character. Or are you saying it's a mistake that she sent her other children off without her? Because it's not... she's trying to protect them. I'm just not seeing what the mistake is here.
Correction: The tune is the same as that from "This Old Man", an older song. Http://barney.wikia.com/wiki/This_Old_Man.
Jon Sandys ★