I notice it more with certain South American and Asian airlines, but occasionally with the smaller domestic (U.S.) airlines as well. (In other words, not so much with the majors. They usually know how to honor a ticket.) It also seems like those who do deny boarding try to do it for other things (interline agreements, billing issues, etc.) rather than admit that it was a fare mistake. I would also guess that frequent and/or assertive passengers are better able to avoid denied boardings like this, just because they know what they're doing or fight it out until they win.

I get probably 3 denied boarding calls a week (much less than average since I work the graveyard shift) and they're usually first-time flyers who don't know what they're doing, aren't assertive, and don't give us time to solve the problem because they show up 1 hour early but don't call us until 5 minutes to boarding.