The Premiere of Star Trek
Five decades ago, six-year-old Dennis Green was channel-surfing (all three channels that were available), when he landed on KMTV, the Omaha NBC affiliate, just in time to see this creature fill the TV screen, all twelve inches of it.
Scared the shit out of me.
It wasn't until years later, when like all Geeks-In-Training, I started watching Star Trek reruns after school, that I realized the creature that had given me a week's worth of nightmares was the Salt Vampire from "The Man Trap." And later I learned that even though "Where No Man Has Gone Before" was the (second) pilot, "Man Trap" was the episode that the network suits chose to air first.
I can't say with 100% certainty, of course, that what I saw wasn't a rerun. But I choose to believe that I watched the Star Trek premiere on September 8, 1966.
At least thirty-seven seconds or so of it.