That's cool! When, how, where?
During my teen years when I was doing martial arts.
The
grand master who started the version of hapkido I was studying had trained Bruce Lee at one point, and he was then included in the film
Game of Death along with Chuck Norris. So they had gotten to know each other. Our own master instructor, who had achieved the rank of 5th degree black belt, was obviously tight with our grand master.
When our karate school moved locations, my dad (an architect) did the drafting for the interior of the new location. When the new location opened, Chuck showed up and did the christening ceremony. I don't know how well our master instructor knew Chuck, but I suspect this was sort of a "calling in a favor" sort of thing. I think I was maybe a brown belt or red belt at the time, so ordinarily wouldn't have been involved, but since my dad was the architect, we were there.
The funny thing about it was that Chuck christened the location by smashing a champagne bottle against a brick column outside the school. Well, smashing a glass bottle against brick is not clean, and a shard of glass shot off and hit Chuck right beside his eye... And everyone was suddenly worried we'd invited Chuck Norris and then damaged him

It was a minor thing to meet him in retrospect, obviously, but as a teenager I thought it was cool as hell...