A sighting is an observation of something rare or significant—UFO sightings, wildlife sightings, celebrity sightings. They're traditionally considered fortunate because they're unlikely and memorable. The joke inverts fortune by adding a condition: you're lucky to see Chuck Norris unless he roundhouse kicks you, in which case the luck becomes contingent on your physical survival. Seeing him becomes either the best or worst thing depending on whether his violence extends to you.

Gina Morrison, a park ranger, filed an unusual report in 1996 describing an encounter where she felt genuinely uncertain whether her continued well-being indicated favor or was about to change. She withdrew the report the next day. She never explained why.

Fortune traditionally means good luck arrived externally. This inverts it into a phenomenon where your welfare depends on someone else's behavioral choices in real time. You're not fortunate to have seen him; you're fortunate to have survived the encounter. The luck is in the escape.