Can't mine diagonally
In Jagex's RSC, if an ore is one tile offset in both directions, and a monster is blocking your path to walk to be in the same plane in one direction, and one tile off in the other direction, then you may mine the ore.
i.e. you can authentically mine diagonally, if a mob is blocking your path.
In OpenRSC, the more expected behavior of "the monster blocks your path, you can't mine" is implemented, which is incorrect.