I never heard about the mew under the truck, just that the truck existed. And it does! The rumor going around for me was that if you held down and A when you were trying to catch a pokemon then they'd have a higher chance of staying the pokeball. I held those buttons with all my might. Turns out that button presses can alter the chances of catching a pokemon, but you have to have split second timing. My technique never worked, but I damn well thought it did.
Also, I knew a kid who claimed, among other things, that you could play as Sheik on Ocarina of Time. Little liar.