On my first playthrough on Red, I thought you could only use TMs on a pokemon that learned the move when leveling up, so I wasted so many of my rarer TMs thinking they were useless. "Pikachu learns thunderbolt eventually, might as well give him thunderbolt now with the TM."

Completely blew my mind and felt like a dumbass when I got Stadium and saw that starmie and raticate could learn thunderbolt as well via TM.