10. The Clock Tower Is Being Unloaded In The Background - Back To The Future Part III
No matter how many times you've seen the Back to the Future movies, you probably missed this terrific secret hidden right before your very eyes.
In the third film, when Marty (Michael J. Fox) and Doc (Christopher Lloyd) are talking to the train driver about the train's maximum speed, pay attention to what's going on behind them.
That's right, you can see a giant clock being unloaded from the train, which is obviously the very same clock that adorns the top of Hill Valley's Clock Tower in the future.
The Clock Tower is of course one of the town's major monuments and plays a significant part in the series' events, with Doc climbing the tower in the first movie to install a lightning rod, which eventually allows the DeLorean to travel through time.
Seeing its origin here is an extremely cute touch, even if the overwhelming majority of viewers totally missed it.