Most buildings are vertical, unless you count the Leaning Tower of Pisa, but time and time again you see pictures where the verticals at an angle. I don't mean the ones where the camera has been turned at an angle deliberately for effect, I mean the ones where 2 clicks in Photoshop would have put it right. I'm hypercritical because when I was training I had my work checked with a set square by a flight sergeant. One of the things I do like about Photoshop is the ability to straighten horizons and lose the bits you didn't notice at the time, or couldn't move in an otherwise perfect shot. I've got into the habit of checking verticals before I even start to work on the content of images.