There are no bad questions...

...but some are better than others.  EM interview season is now in full swing, so something interview related makes the most sense for a first post.  Just today I was asked for one piece of advice for someone going to an interview.  My answer was to have "good" questions.  Sounds bland and useless but I really think this is important.  If you get me rolling on your questions I may never get around to my standard list of tough questions.
So what is a "good" question?  This is best summed up as a question that truly matters to you.  Interviewers often get generic, safe questions, answerable by perusing the website.  Less appealing are the "stalker questions" targeted to get the interviewer rambling about their professional interests and activities.  If you want to create a favorable impression with the interviewer, challenge them with a question you actually want to know the answer to.  Ask about something you are passionate about.  Or something you are really concerned about.  Your genuine interest will show through in a way that does not happen when asking about something you have been told you are supposed to ask about.  And it is perfectly fine to ask every interviewer the same question.  You may get some insightful variation in their answers.  
Nothing shows disinterest more than having no questions.  Generic is certainly better than none at all.