This reminds me of MI2. I'm not really "proud" of this but oh well.
I had found the largo's ancestor grave on Scabb island and knew i needed to open it, so i went looking for a shovel or something to dig it up.
I spent about three month trying to figure out what i had to do. I even stopped playing the game out of frustration, i'd just launch it now and then and do a little bit of "let's just click everywhere vaguely hoping something will happen" for a few minutes, then got pissed again and shut it down.
Until for some reason i decided to read the manual. I was bored and i thought lucasart's manuals were often funny, but i hadn't bothered reading that one yet.
And there, i saw a screenshot with a shovel in the inventory. There was even NOTHING but a shovel.
I started reading, and, as they were explaining the basics of the game, they were telling you to "pick up the panel"... IN THE VERY FIRST SCREEN OF THE GAME

I had seen this "treasure huntery forbidden" or something pannel over a thousand times, and had always assumed the damn thing was just a drawing


So yeah, i felt more dumb than proud really, but god, this was quite a relief (luckily my three months struggle was rewarded with one of the game's funniest cutscenes

)