Well that would depend heavily on the type of massage.
I would say that most health conditions are not at risk of being exacerbated by light massage.
When you get into deep tissue and heavier handed massage I would say possibly really high blood pressure might be an issue, but even that would seem to be a rare thing.
I once had a case where I was massaging a client with cerebral palsy, repeated shoulder dislocations, and many other severe issues. I was in fear at first, but did nothing but help.
= )