When I found a position that enabled me to leave my last job, I was incredibly excited. Now I'm interviewing for my next position (due to major life changes, my current commute is no longer manageable), and I'm fielding the question "Why did you leave your last position?"
Here's the true answer: My boss (the CEO of the startup I was working for) was impossible and made my day to day experience nerve-wracking and stressful. He shouted and screamed at employees and customers alike. His expectations were unmanageable and he didn't trust his employees in their areas of expertise. I couldn't explain enough times why maintenance tasks needed attention, he felt that my time should be spent 100% on the creation of new software/features but also insisted that bugs (which were numerous due to massive amounts of untested speed-developed code from the developers before me) be handled immediately. He fired employees for looking at him wrong (not actually for looking at him wrong, but one employee was fired for vacuuming mid-day after we were asked to pitch in with cleaning). I don't dare let a potential employer call him for a reference, despite having left on fairly good terms (as good as I could expect), as I overheard him "give a reference" once where he tore the former employee to shreds, complete with expletives, and explicitly stated "Do not hire him." Additionally, he employed an accountant, who was also his mentor, who felt that it was appropriate to not only shout at employees, but to belittle them in front of each other.
I have no idea how to phrase this in an interview when I'm asked why I left. I know it's bad form to say bad things about a prior position. I feel I handled leaving that position as professionally as possible, submitting my resignation quietly and in person, giving time to wrap things up, documenting all of my work, and even helping to hire my replacement. But it's continuing to haunt my career!