1) You just got your first job and you are right out of school. They call you a "programmer" or "analyst" etc, but you don't know the profession or business and don't know what you're capable of. You'll outgrow it by hard work and learning.
2) The feeling many experts have after a long career that "any smart person could do this if they had the time and inclination". It's usually not true (because they overestimate capabilities of others, assuming they are like them), but I think it feeds into the humility you often see among the truly capable.
3) People who have remunerative yet non-productive jobs where you can get paid lots of money but your production is very abstract or secondary or even tertiary (or worse) to your direct output. This is the kind of person who talks about imposter syndrome at TED talks.
I have to think that what we call "imposter syndrome" is, in at least 25% of cases, and extremely useful internal guide that you need to step up your output. The solution is to do work where the direct output is something useful.