When I was still at tech school, I brought my 94 corolla in to do a front end alignment. Set it up on the rack. Set the sensors up, proceeded with the alignment. I got everything well within spec, but upon taking it for a test drive, it had a crazy pull to one side. I had my buddy who was helping me out test it. He agreed, it was very noticeable. It didn't have mad pull before. I thought "how can this be? The toe was well within spec." nearly perfect when I got done. What forgot was to center the steering wheel and use the steering wheel lock before I started making adjustments.

Did it over, with the lock, and it drove perfect afterwards.