As a setup, we know that Marty came back to see Doc right after the clock tower incident, which was on the night of November 5. In the scene where Marty and the Doc are at the Drive-In theater, getting ready to send Marty back to 1885, we can see on the Time Circuit Display, that the present time is 11 05...which is a slip-up in itself because it has to be at least November 6. Even if the display is acting up, and it's not showing the correct date, how did Marty get the picture [that he took the night before] of Doc's tombstone developed in less than a day?
Now, I'm no expert, but I know that Marty didn't use a Polaroid Instant camera (the one that spits out a picture as soon as you take it) and I'm pretty sure they didn't have 1-hour photo development processes in 1955. For that matter, how could the Doc have purchased all the parts he needed to fix the time machine and assemble them in that same time span?
On top of that, in the same shot, we see that the last time departed is 10 26...and even though the year is difficult to make out, that date is wrong because the last time departed should be 11 05, when the flying DeLorean was sent back to 1885.
One final slip-up in this same scene is the use of the Time Circuit Display: in all other shots of the display, the month is represented with a three-character abbreviation of the month name, but in this shot, it is clearly a two-character numerical representation of the month.