Question: Could a massage session do more harm to my body than good, and how would I know if something is going wrong?
Answer: It's pretty hard to do any harm with Swedish or the more gentle massage, so pretty much anyone can receive that without any problems.
As far as the deep tissue massage, there are a couple issues. One is that we rely on the skill of the massage therapist knowing just how hard to press to be helpful to a person and not to flare up their pain.
The other thing is that it's important as part of massage therapy training that the therapist know the contraindications to massage -- in other words, if someone has a condition like a fracture or cancer in an area, then the therapist is going to avoid doing anything where there would be deep pressure in that area.