Back To The Future - Effects in Time...
When Marty is up on stage at the ENCHANTMENT UNDER the SEA DANCE, (Playing a guitar made 4 years later!)the proper distortion & effects to play "Jonny B. Goode" in that manner were not yet made until somtime in the near future. Unless afterwards Marty went back before the dance and left a distortion pedal on stage- but thats a different story.
2 eyes, a copy of this film, and a computer to read this...