Oh, well, I have a few ones... 99% of them being on emulation playthroughs because of never being able to get an actual console u.u
1) I always play while pressing the fast-forward key. I was leveling my pokémon up to 100 on Silver, OHKOing everything on sight when I find a Rapidash. My Red Gyarados quickly KOes it with Surf, and I stopped pressing fast-forward for an instant... only to see that the flame was of a different colour. Yep, I KOed a shiny just by not bothering to pay attention to it earlier
2) On Emerald, having a Huge Power Azumarill that only knew special moves. Then I wondered why Surf did almost nothing to Phoebe's Dusclops...
3) By looking at Guts's original description in Gen III (That Attack increased "when suffering"), I purposely left Swellow with very low HP for quite a time
4) Not knowing STAB even existed until pointed out in Gen IV
5) Thinking Glalie was Ice/Dark after seeing "It's super effective!" after using a Fighting-type attack on Glacia's. Somehow, I never thought Ice could be weak to Fighting. Likewise with thinking Steel was weak to Water after battling with Jasmine's Steelix. YET AGAIN when thinking Flygon was Dragon/Flying after Earthquake failed to work (Though it was not noticing it had Levitate)
6) Thinking Magic Guard prevented poison damage outside of battle while playing on White. Then I came across the fact poison no longer did that on anything
7) Blaze Black 2's very difficult Challenge Mode. I had a lot of troubles beating Roxie's team (Dat Koffing with Flamethrower >.<) and after reading the guide, stating that Victini's event was available from the start, I caught it hoping it to steamroll Burgh's team. "The foe's Masquerain used Hydro Pump!"
8) Factual facepalm. Found a shiny Spearow, that being my third random shiny ever (The aforementioned Rapidash, and a Zigzagoon), on Emerald. On RANDOMIZED Emerald :(
9, 10) The last one counts twice. Long ago, in my very first Silver playthrough, I got stuck after beating Falkner as the translation was pathetic and I didn't know I had to pick Togepi's egg. Much, much later, about two or three years ago, my brother got a (Completely mute) Game Boy Color from a client in where he used to work, with a few games, Pokémon Silver included, as I really wanted to play Pokémon for real. So I put the Pokémon Silver cartridge and turned on the GBC. After starting a new game, I noticed the text was very strange, and when I saw only five characters on all pokémon's names, I realized it - it was the SAME pathetic translation I got stuck on as a child. Double facepalm