So long as boundaries are kept then mild flirting is harmless. In general it's just a form of friendliness taken to a higher level. I always thought it meant you were hitting on the person, but I've learned that mild flirting is not at all. It's just a way to keep yourself feeling alive. However, if you take it to a sexual level, then it is inappropriate in therapy. It's just a gateway for misinterpretation and hurt feelings.