When I was young and playing Pokemon FireRed, I was convinced that Moltres, Articuno, and Zapdos were the only legendaries in the game.
Despite the fact that I knew what Mewtwo was, I still used my master ball on those three exclusively.
On one run, I remember catching all three of them without a master ball, and celebrating by catching a Pidgey with it.
Another stupid thing: I was deathly afraid of Mewtwo because I misheard what one of my friends said as "he will be on cinnibar island at level 1000". I remember getting my Charizard to level 80 using only the VS recorder and Route 13 before resetting my game with my OP pokemon why!!!!!!!!!!!!!!!!!!!!
I eventually figured out he was talking about something else in Pokemon Mystery Dungeon: Explorers of Darkness instead of FireRed. Drat.