When Marty looks up the Doc in 1955, Marty is showing him a note that he got from his girlfriend Jennifer in 1985. On the back is the "save the clocktower" advertisement. The Doc spots the ad and comes up with the idea to use the power of the bolt of lightning to get Marty BACK TO THE FUTURE. But when he takes the piece of paper away to look at it in the light, you can clearly see that there was no note from Jennifer on the opposite side of the sheet.