After signing a contract to accept a tenure-track job, you should not subsequently back out.
I now know of two people who have done this. And seriously, dudes, what's so hard to figure out? If you weren't sold on the institution, you shouldn't have accepted the offer. If you were waiting to hear from another school where you had a campus visit, you should have told the offering institution that, and asked for more time. But if you thought you were out of the running someplace else, and then they came knocking--or if a fancier job appeared in the spring job list and you applied anyway--you kinda suck.