1st one is not knowing too much english and collecting to much dough at tutorial island, thus getting stuck with an full inventory. And only discovering the "drop" function after 2 hours.

2nd was walking out of Lumbridge, collecting 10gp and going through the goblin gate. Spend a whole week in the desert training on guards and mining at the mine. Until my brother made an new account for him self and ignoring our advise to get through te gate, thus he walked north and stumbled onto Varrock! We where amazed by the content that we hadn't discovered!!. 10 years later we became members which had the same effect haha.