Hm, well, an obvious one would be fighting Brock with only Pikachu, since I didn't know about Mankey/Nidoran The worst part is that I did this TWICE, since I made the mistake of letting one of my neighbors play my Yellow version and she explicitly overwrote my save even though I told her not to -_-
Other than that..I remember there being a rumor in my neighborhood that if your Pokemon reached level 100, they would die and go to the Pokemon Tower or something like that. Part of me knew that that was stupid but at the same time, since I was little, I was worried that it was true. Of course none of my Pokemon reached level 100 until B2W2 and by then I knew it was fake XD
I did the same thing as Pink!, and I tried to fight Brock with Pikachu. I eventually won, take that Brock.