When Marty first goes back to 1955 and eventually meets Doc, Doc doesn't believe Marty is from the future. When Marty tells Doc that Ronald Reagan is the president, Doc is astonished. A couple of lines later, as he is running out of the house, you can hear him yell, "I suppose Jane Wyman is the first lady!" Jane Wyman was Reagan's first wife, true. But Reagan had already divorced her and married Nancy Davis in 1952. So, theoretically, Doc shouldn't be making an assumption (even though he was talking out of disbelief) that Jane Wyman would end up being the First Lady. This line was put in obviously to make the situation seem more authentic to 1955, but the writers "slipped-up" and didn't get their facts straight. Since there was already Nancy Reagan in 1955, the line shouldn't have been there at all. This is a definite slip-up.