In the Fringe episode "Peter", we see a flashback with two observers coming out of a movie theater showing Back To The Future. However, in the alternate universe, Marty McFly is played by Eric Stotz and not Michael J. Fox.
In one of the most amazing nerdy trivia Easter eggs so far in Fringe, it turns out that the role was originally cast with Stoltz, but after filming began he was replaced by Fox.
Here is a clip explaining the Back To The Future lead change:
* Thanks to Todd F for this amazing tip!