In Back to the Future Part II, when Marty and Doc return from 2015 to 1985, Marty asks Doc:
Did we make it? Are we back?
Why did he ask that? Did he have doubts that the time machine was working? They had used it successfully a couple of hours before to jump into 2015. What could have been wrong with it?
I know this may sound as a silly question but still it disquiets me.