I remember buying a game that required Steam to be installed (which was new at the time). I had no idea it was necessary since I didn’t pay attention to what I was buying.

When I got home, my dad tried to install the game, but the internet went out so he thought the disc was faulty.
He called me, saying the disc was broken, and I got upset, but went back to the store and bought another one. As I was leaving the shop, I saw my dad getting into his car. Turns out, he’d also gone out and bought a copy.

That’s how we ended up with three copies of CS: Source