I had a business meeting with some UA folks several years back. Among the activities was to understand the boarding process. So we went out to some ORD gates and (pre 9-11) tried our hand at being boarding agents (it was fun telling people 'Welcome Aboard' and so on), shadowed by an experienced agent of course.

My trainer told me of an incident where she was to escort an elderly lady through her connection. She was mortified when she found out she had boarded her on a plane to Manchester, NH instead of Manchester, UK (after the plane was in the air of course.) She did say that those kinds of errors were a lot less likely with the boarding pass readers