Perfection is unattainable but we strive for it anyway. I actually took my time and tried hard to center this photo dead on. And I failed. You might not notice it if you just glance at the photo and move on, which is what most people do. But if you look, you'll see that there is slightly more space between the column and the front door on the left side of the door than on the right; and slightly more space between the inner column and the window on the right than on the left. Blast.
Perspectival distortion (in this case, the keystone effect) was corrected in Lightroom 3. DxO Optics Pro is even better than Lightroom for this kind of thing but I was able to do well enough in Lightroom. The correction didn't introduce those slight variances that I talked about above. Those were simply a mistake at the moment of capture.
I really wanted to stand about six feet closer, but that would have put me into the middle of traffic.