Irony is defined by the American Heritage Dictionary as "incongruity between what might be expected and what actually occurs". Irony can be found at different places within the Back to the Future trilogy:
In Part I
- Marty McFly, who becomes the first human time-traveler, has problems with keeping a working timepiece.
- Regularly late for school, Marty is tardy four straight times by October 25, 1985, but then arrives almost 30 years early for his next visit to Hill Valley High School in 1955.
- Biff Tannen, who had totaled George McFly's car in the original version of 1985, ends up taking care of George's car (as an automotive detailer) in the revised timeline.
In Part II
- In the alternate reality of 1985A, Biff's Paradise Hotel and Casino is the center of a place that Doc and Marty compare to being in hell.
- Marty erases 1985A and Biff Tannen's wealth by burning Grays Sports Almanac, using a book of matches from Biff's hotel.
- Blast From the Past, which in 2015 specializes in things past (as an antique store) is in the building that in 1985 had been The Third Eye, where a fortune teller specialized in telling the future.