Confusing values
You can make many bad decisions by assuming that other people will share the same values as you do in every circumstance.
Consider: the employer that values his job security (and therefore deniability) more than your work ethic, likeability, or integrity.
The employer who cares more about your track record than the passion (or ideas) you'll bring to the job.
Or, even, the employer who will give more weight to your performance in a job interview than anything other form of assessment.
The lesson: Expecting that people will see things the way you do and behave as you'd expect, in every circumstance or context, is a surefire way to be as bewildered as you are disappointed.
On the other hand: treating different people differently--not as you'd expect, but as they are--is a great way to be a little less surprised.
Because people aren't stupid. They just see things differently. For better or for worse.