Alot of this adds up to patterns.
Frequent Job Changes
There's times that it works, and times when it doesn't. I'm not going to define "frequent" - it varies by location, industry and current market state... but in general:
Contractors and Known Temporary Work - never really a big deal. This generally a job choice, and people who like lots of job change are good fits for this. The corallary is that anyone who does lots of short term work is expected to have mastered the techniques of ramping up fast, getting work done and doing closure well at the end of the project.
Small/Risky Companies - my general experience has been that those who prefer smaller/riskier companies tend to have a shorter time frame in any given company. Small companies themselves come and go quickly and people who work in this area view their careers more as an evolution of skills, not a growth within a single firm. Abrupt ends to jobs or leaving because of extremely risky financial status is not unusual.
Bigger/More Stable Jobs/Companies - the bigger a company is, the more it will look for employees with a tendancy to long-term commitment. A massive company isn't likely to change (or die) very quickly, and there's usually a tremendous organizational overhead to be learned. This is the area where there is a really low return on investment in the early months, and so it becomes very important to keep people around. While no longer the norm across most corporate culture, the biggest/oldest companies are also where you will the "lifers" with 20-30 years of experience in the company, so that the "new kid" has only 10 years.
Trends vs. Uncontrollable Choices
There's plenty of cases where an atypical choice will occur in any job history pattern. When starting a new job, there are a ton of unknowns and plenty of unfortunate situations that can startup. Cases include:
Hidden financial status - no one told you this company was going under, and while financial information is reported for any publicly traded company, the link between the real accounting status and the one presented on paper is often deliberately obscured. Accounting isn't as black and white as one might believe - just look at some of the great financial scandals.
Hidden Jerks - whether you misjudged your boss, your team, or just didn't know what this group was REALLY like, there are conceivable cases where you just need to get out. I never advocate this as the first thing to do, generally rising above this sort of adversity builds character, and "jerk like behavior" can often come from a need to change a communication pattern, something you yourself can control. That said, I can conceive of cases that so extreme that I can understand that "stick it out" isn't a universally good plan.
Job not at described - there are ranges, particularly in knowledge work, where the job description is QUITE broad. But I'm willing to believe there are some really extreme cases here - for example, getting hired for a skill set you don't have and never advertised about yourself. It happens.
In these cases, I say be aware of trends. Coming into a singlular mess and looking for a new job very quickly thereafter isn't such a career killer. It's not unreasonable to say, in these cases, that the job is not at ALL what you expected.
The thing I watch for is a trend. Regardless of the reason, when a candidate comes in with numerous short time job changes, I have to ask "why the trend?" In contractors and risky company lovers - the trend is legitimate - it's a context of the work. But when talking to a candidate that says he wants a long term, stable situation, and seeing a case of many job changes - you wonder what's going on. Working through adversity is part of most jobs, so a trend of leaving when the going gets tough makes one wonder what will happen if it gets tough in this job.
Going Down With the Ship
These days, there's no 100% answer here. Quite often, individual contributors with serious family commitments and an inability to tolerate financial risk will leave when a company gets into really bad shape. Being in a company going through a mass exodus is pretty obvious - there's been many bad financial quarters, the best of the best start leaving and the work becomes grueling and unpleasant. It's not usually an expectation that you stay, although I've seen cases where smart companies will offer great options to those who are willing to take the risk and stay. It's got a lot to do with the personal tradeoffs involved.
I'm not even sure that these days management is expected to stay - particularly in a large-scale organization, the various levels of management can be so deep and so removed from the ability to affect change, that staying on the sinking ship won't change anything.
I'll say that at a certain level, a manager or executive is expected to be accountable for the decisions and how they affect both the company and the people. A manager or executive develops an industry reputation, and how you act in a crisis is a big part of that. Obfuscating the state of the company's finances while looking for a new job is probably NOT the reputation that most managers want to build. Being honest and caring, even during a layoff - is the mark of a good manager.
I'd put it in more positive terms, though - being the guy who was there to "turn the lights off" - is often a mark of disctinction. It's like being the last kid "alive" in a game of dodgeball - you clearly had to have some skills to survive in that environment. If you're willing to take the risk (and you can, with your personal constraints) - you might just learn a LOT which makes for a great sell on the next job.