When I started gaming (along with some other players that were new to the game), our DM immediately reminded us about one of the biggest differences between the tabletop version and the computer version: "No, you DON'T find random objects or gold pieces in every single box/drawer/barrel you open!!!"
I completely misunderstood the concept of game-time when I started and thought it was the same as real time. I used to bring a stop-watch to game sessions, to measure how long combats took ("Hurry up, man! The round is almost over!") I'd get caught up in the game, of course, and continually forget to look at the watch and realise I had lost track of what round or turn we are on. Eventually I decided that this approach was not just crazy, it was unplayable as well. So I went back and re-read the section in the Basic Set on timekeeping in the game.... oooooooh, I seeeee......
First time I (or the DM) ever played the game we were playing out a tavern brawl that spilled into the streets. One NPC pulled out a wand of Fireballs. The DM looked up the effects and said, "It CAN'T do THAT much damage! NOTHING would survive in the entire area of effect!" He then proceeded to houserule a series of damage rings where only the individual at the center took full damage. Each 5' ring beyond that took a lower number of dice of damage - in addition to getting your save for half. It would be a while before he'd realize that 1st level NPC's should not be using wands of Fireballs.
Most of my own "preconceived notion" errors are just mispronouncing a few words I'd never seen before because unlike some people I actually pay attention.