Which then in most cases is too late, and you're left in the lurch behind the 8-ball.
Case in point - Target's acquisition of the Zellers leases on their locations.
I had over a dozen people I know who were offered pretty lucrative jobs there. I know another half dozen who balked at their offers (including myself). The people who went were only there about 6 months. Right away, there were signs this was doomed.
For several months all I heard and read was stories about how this was not going very well at all. Two of my friends who took jobs started looking as rumors swept through their departments. They both found employment in short order, right before the roof collapsed and Target announced over 1,700 layoffs.
If you decided to wait it out, you were now entering a market were you had a lot of competition for your same job at any number of businesses in the area. Thus, reducing the likelihood of finding something quickly. I have several friends who still haven't found a job six months later.
I went through the first dot com bust with two startups. Because of that scary experience, I've always been leery of my continuing employment anywhere. I'm become somewhat adept at reading signs and realizing when something is about to go tits up. As such, I interview constantly, and always have an exit strategy for any position I take, whether its a perm gig or a contract gig. I constantly look over my shoulder and its served me well.