When I was younger I used to pronounce 'premise' as 'premies' - "mies" rhyming with "pies". I didn't realise I was pronouncing it wrong until a friend pointed it out, and I was embarrassed over it.
I've had a few other mispronunciations before...
Only time I got seriously scared post-childhood was when I was playing Dark Souls III and there's this sewer in one of the world and after you explore it a bit, a GIANT FUCKING RAT rushes at you from behind the fog. The combination of the sheer...
Oh there are so many things lol. There's the Luigi's Mansion Beta like @Really Ilini said but also slightly creepy thumbnails I'd come across for a split second (yes that would prevent to sleep for a day). There was also the Sonic CD Majin Sonic...