I'm surprised no one has mentioned this MAJOR plot hole in Back to the Future II:

You can't visit yourself in the future.

Okay, so there is that pesky suspension of disbelief thing, right? I'd be willing to buy that argument except: The parameters for traveling into the future were already established in the original Back to the Future. Got your copy of the movie ready? When Dr. Brown first demonstrates the time machine for Marty (who is videotaping), he has his dog Einstein get into the car and sets the time circuits for one minute into the future. After the car hits 88 miles per hour, it vanishes - traveling into the future. Now for an entire minute, where is Einstein? That's right - he's gone! See, that's how traveling into the future would work, and how it is shown in the first movie.

Back to the Future II couldn't happen because when Marty and Dr. Brown go into the future, Marty would no longer have a family - he would have been missing since the day he time-traveled from.

This creates the largest plot hole of all for Back to the Future III, the movie that came before it couldn't have taken place at all.

Since adding this node, another node has been placed to dispute this theory. While it doesn't bear repeating, the mere intention to see yourself in the future does not give you that ability. Einstein's trip was instant to him because he was in a device that allowed him to "skip over" time; however, since he was INSIDE that device - he was no longer present as time continued for everyone OUTSIDE of it. The REAL plot hole in Back to the Future II is the changing of time travel theories. In the first movie, time is linear and the time machine enables you to move within that line (Much like in the H.G. Wells The Time Machine novel). In Back to the Future II, the theory changes to alternate realities.