One side of the cake rises higher than another
It's probably not due to uneven oiling of the pan. 

Assuming you haven't managed to stir your batter so that the baking powder (or other leavening) got distributed primarily on one side of the cake, which sounds pretty impossible, the most likely source of asymmetry would be the oven. Are you sometimes cooking these cakes too close to one of the walls of your oven - I mean off-center in the oven? 

Or maybe there is an uneven temperature pattern in there. Not sure how to remedy a situation like that, except for trial and error and noting the positioning when the cakes on occasion do come out even.