When I decided to accept a job offer, the thing I hadn't prepared for, or even thought about happening, was having to respond to interview requests from other colleges. It was one thing to say "I'm considering an offer from another college"; it was quite another to be faced with saying, essentially, "I know my cover letter made it sound like yours was the only college for me, but actually I've already accepted a job somewhere else."
How does one phrase this. It's not like you can say "Unfortunately, I've already accepted..." because it's not unfortunate. I'm not about to express regret about something I don't regret at all, but I don't want to sound like I'm completely brushing the invitation off. It's an exercise in tact and delicacy that I've had no training for in graduate school.
Fortunately, it hasn't happened often. Within a few days of accepting the offer, I received one invitation to an on-campus interview - very awkward, because that college called me instead of e-mailing, but the close timing meant that I could honestly say that I had just accepted another offer, and let them read into that what they would. Later that week, I received an email for a phone interview, and after agonizing briefly I just said "Thank you for the invitation; however, I have already accepted a job at another college".
It seems like the academic job application process is just set up for this. Dozens of applications are sent out, responses don't come for months - and, as it may turn out, all in a rush - and who can say what will have changed in the meantime? I would feel incredibly arrogant informing the colleges I haven't heard from that I'm withdrawing my application, but instead I'm left with the mild shame for not considering their offer. At least it isn't likely to happen again.