People back out all the time. It's more a question of backing out gracefully and not making it a habit!
I agree with this one - its all in how you back out, and the circumstances around the back out. And, not making it a habit. The job I'm at now I got totally unexpectedly, after giving a tentative verbal 'yes' to go out as a replacement on a tour.
I had inquired of the touring company if they knew yet whether they would need a replacement for the spring (it was a company I had worked for for several years and had a good rep at) and as of the Fri of that week, their person wasn't going to quit (he'd been threatening). So, I did some job hunting over the weekend. On Mon. things had changed - the guy did give his notice, and they wanted me as the replacement, and I said 'yes'. (this was all during the tour's hiatus) Well, the Thurs of that week I found out that I was on the short list for one of the companies I had sent resumes to over the weekend. Now this was a case of debating between a 6month tour contract with a company that while they liked me, I was ambivalent about them - to a full-time, year-round, salaried with benefits position at a road house that paid more. I couldn't pass up the offer.
I was honest with the touring company, and they agreed that I couldn't pass up the other offer. I helped them get ready for the spring part of their tour, doing some of the prep work that their departing person hadn't done, and I have been helping them with info on the venues that they're going to. If I ever decide to leave the new job, I will be welcome back at any time there.
It does bring up a crisis of conscious though - and I think that anyone who does it on a regular basis should be looked at twice. I was horribly torn between the two commitments when I found out that I was being considered for the second job, and I really hope that I never have to be in that position again.
-Centaura