From the Butterfly of Doom to the Temporal Paradox, this film, despite it's less-than-stellar reception, completely validates everything Doc Brown warned us about when it came to time travel. Specifically, that even the slightest changes can have disastrous consequences for the future, and that meeting either your past or future self face-to-face is a very, very bad idea indeed.
WARNING! Spoilers are unmarked.
- David spots his present 17 year old self on a tape recording from his seventh birthday party back in 2004, ten whole years earlier. Understandably, this freaks him out quite a bit.
- What freaks him out even more is that he knows he isn't seeing things, because his friends can see him on the screen as well. They try to come up with a rational explanation, but they can't. Somehow, David is there, on a video recorded ten years ago.
- Whenever someone directly encounters their past/future self, the effects are pretty nightmarish. Rather than being able to have a conversation with your other self, both will start parroting each other and then begin to flicker in and out of existence due to the paradox as the timeline attempts to fix itself. Eventually, if neither version breaks eye contact with the other, the timeline will straight up erase both of them from reality, with the results being seemingly permanent. The only way to fix it is to go back even further and prevent the two selves from ever meeting.
- Time travel itself in this film is quite bizarre and much more dangerous than most other examples. The process of travel is shown to be rather distressing, and the Butterfly Effect is constantly in play and is lethally unpredictable, with even slight changes appearing to have major consequences for the entire world. For example, David screws up his one chance to be with his crush. So he decides to travel back and kiss her, making it so they're together, with no foreseeable harm being caused by this innocent act. That one simple trip into the past to change something that had already happened through a kiss ends up causing an airliner to crash, alongside several other disasters. When he goes back to fix that, his friend Adam ends up being hospitalised. David then keeps trying to fix things and only ends up making it worse every time he jumps back. The scariest part is, we never even find out how he ends up causing any of it to happen.
- The group decides to send an RC car with a camera and timer back one minute in order to test the machine for the first time. The car instead ends up going back two hours and becomes fused into the basement wall. Not so bad given the circumstances, but now imagine what would have happened had they not decided to try out the machine on an inanimate object for their first test and just sent one of themselves back instead? Would it have been the same result? Or would it have been much worse?
- Not to mention that if whoever went survived the trip, they would most likely encounter their future self and cause a paradox that would make them disappear.
- David's father was working on the time machine for a government project codenamed "Almanac". Considering he hid the time machine just before his death in a car crash (and given the seemingly classified nature of the project), one has to wonder if his father was assassinated because he wouldn't give his employers the time machine.
