Marty's Tardiness
Marty is habitually late to school in the present-day, yet arrives to his high school 30 years early with the help of Doc's time-traveling DeLorean. Now, a kid that is usually late to school is incredibly early to school, something that would not be expected.
Energy
The DeLorean needs plutonium to use as energy to power its time-traveling, yet lighting -- something very dissimilar to the material -- can be used as a substitute. This is totally unexpected and surprising.
Strength
George is portrayed as a weak and timid man, yet he is physically strong and capable of beating up other big and strong men.
Destruction
Biff Taffen totals a couple of cars in the film (most notably George's) yet works as a used car salesman in the future. It's surprising to see that someone who showed such distaste for cars worked to sell them and not destroy them.
A Time-Traveler Who Can't Keep Time
Marty is the first-ever time traveler, yet he has trouble operating and keeping time with his watch. Audiences would expect someone who breaks such a revolutionary barrier to be at least proficient in time-keeping. Marty was not.
