Oh, this is a tricky one. It's a setup. It is very easy to fall into this snare. People just wait for an opening to vent. But you now know ahead of time NOT TO DO THIS.

Instead of coming back with, "OH, I really hated my boss. He was such a pig!" you want to go to a task that is harmless to what you are trying to achieve. If you worked at Starbucks you had to clean, you had very busy times, it could be stressful, people complained, you may have had a bad co-worker, etc. But if you point any of this out in your interview it will look like a negative for YOU.