Anecdotalism
Just because “this one thing” happened to “a guy [you] know” does not make it universal. You might be describing an edge case or once in a lifetime situation. It’s not valid to pass that off as evidence that something is universally true or false. Repeated instances of the same thing happening again and again are certainly evidence of something, but a single anecdote proves nothing.
Examples
Why We Need Science: "I saw it with my own eyes" Is Not Enough
Seeing something with my own eyes doesn’t prove it’s true and it doesn’t preclude the necessity for scientific testing.