The point in a job interview where the tables are turned and you are asked if you have any questions can sometimes be challenging. If the person interviewing you is also likely to be your boss if you get the job, one good thing to ask is how they keep track of the projects they're responsible for managing.
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That question was suggested by an audience member at a Linux.conf.au 2011 presentation by Google's Carol Smith on how to more effectively manage projects. One of the key issues she identified as a problem for many project managers was an obsession with constantly checking with employees on the status of individual tasks. That kind of heavily hands-on boss can be difficult to work for, so asking an interviewer their approach to project management might give you an idea if you really want to work for that company (assuming you have the option of not taking the job if it's offered).
A second question suggested by the same person is to ask the interviewer when they last took a holiday. While that will help you identify if you're potentially going to have work/life balance issues down the line, it's potentially a riskier move, since it might create the impression that you're overly focused on taking time off.