Private access roads that distort routing 
Many new editors enthusiastically start mapping all small streets, alleys, driveways, parking lot roads, etc. They think "if I can drive on it, I should map it". But this can have some negative effects. Of course it means a lot of extra data (bandwidth) and extra calculations. Also it can make the map less clear on small screens. But the biggest disadvantage is that it can distort the routing.