possibleI don't know WTF my problem is, but my horizons are always off. I even consciously think "straighten your horizon", but it still comes out crooked. I think it's my tripod, the thing is junk.
possible
2 other things...autofocus viewfinders make it almost impossible to line things up properly since the points are so dim.
also, you may have a tilted sensor/viewfinder. my camera does...so even if i line things up straight, they come out crooked. it's cause the sensor doesn't line up with the viewfinder...