Affirming the Consequent
The Fallacy
Thinking that just because you know the end of a story, you know exactly how it started.
Why it's wrong
Just because something happened, doesn't mean it happened for the one specific reason you're thinking of. There could be lots of other reasons!
Example
If I eat too much candy, I get a tummy ache. I have a tummy ache right now, so I must have eaten too much candy!
What the Fallacy?! (ELI5 Edition)