Other mistake: Just before Marty starts playing Johnny B Goode on stage, he tells the band members to watch for changes and other guidance advice to start the song. When the song starts, ALL the band members (who have never heard this song before) suddenly play all the chord changes perfectly and even the drums are perfect like the real song. In reality, never hearing that song before, the band would not have had this song perfected like that - every note and instrument sound.
Suggested correction: This is, of course, done intentionally to make the song sound like it actually is. It's highly unlikely but not impossible that these musicians put it all together just as the actual song sounds. You also have to keep in mind that the idea is given that Chuck Berry came up with the song hearing it through the telephone, so that's why the actual song sounds the same, as he is just mimicking what he heard.
Besides which, if this posting were valid, every movie musical in history would be riddled with similar errors.
Continuity mistake: While Marty is at Lou's Cafe, Biff and his gang enter. When Biff starts bullying George, note George's haircut with shaved sides, back (above his neck), and sideburns. In the following shots, George's haircut with its shaved areas change length. This happens again, when George is with Marty and then tries to ask Lorraine to the dance. (00:39:00)
