1) I spent about 10 minutes trying to work out how to exit the house I started the game in.
2) I didn't know about Pokemon centres, so every time all my Pokemon fainted, I'd start back in Pallet town, and I'd have to walk all the way back to Cerulean City or wherever I was.
3) I'd spend lots of time trying to extract Mew from under the truck, and I also tried looking at Bill's page for "Eevee" 50 times because I thought he'd then give me a "Pikablu." And I thought "MissingNo" was supposed to be a real Pokemon.
4) I couldn't find the "Card key" in Silph Co; consequently I couldn't get the Marsh Badge. I then tried sneaking past the guard in the Victory road gate by swimming around the long way.
5) I didn't know about "flash" so I repeatedly tried making it through Rock Tunnel in the dark, sometimes ending up back at the entrance instead of the exit.
1) I tried to get "Mew" by breeding two Mewtwos. :P
2) I didn't think that trading would auto-save, so I tried restarting Pokemon Red to trade the other starters across, and as a result I lost my original save file.