A guy I knew did too well on his aptitude test to become a tram driver here in Melbourne. The problem as seen by the recruiters is that in being too bright, he's likely to get bored sooner and leave. Sure, he himself might absolutely love the job and stay for decades, but the recruiters are playing the numbers game here - tram drivers are not a keystone position - and they find that there's more churn with really bright people.
That's a good point, and it's often quoted to explain the fact that Ph.D. graduates often make less money (or have higher unemployment rates) than professional degree holders in the same field. The assumption is that the Ph.D. holder will jump ship at the first offer of a job at his educational level, so he's less reliable as a long term employee.
