Not a sign of a bad manager, but kind of a pair of horror stories:
Twice in the last five years I've been hired by a manager I got along great with during the interview process, only to be informed during my first few days that this person had resigned and his departure from the company was imminent. In my current role it meant that I was the lone person on my team left in the Boston office, with the rest of the team in an office thousands of miles away. In a role I was hired for five years ago, I was brought into a role I didn't have the skill set for, with the promise that I'd be given training to fill it. With the manager's departure, that promise was broken, and it was obvious that his replacement, who arrived some months later, expected someone in the role who actually knew how to do the job, and had no desire or intention of training someone.
I can hardly blame someone for leaving for greener pastures, but, both times, it's certainly left me in a difficult position. I'm someone who values good working relationships with people, and, in both cases, the loss of promising relationships with a manager has subtracted significant appeal from the job.
On a serious note, how do I avoid this in future roles? After the first time it happened, I joked that I should ask prospective managers, "Do you still see yourself at this company in six months?", but now I'm seriously thinking about asking that.