It's not that you can be injured, but you can certainly feel worse the next day instead of better the next day, because of the fact that sometimes we are going in so deep. And if a person does go in too deep and you allow them do that, then you may be bruised or something like this, but a there will be no permanent injury.
A lot of times people will feel worse after a massage, especially the deep tissue massage because of the fact that the therapists are going in and breaking down some of the areas of the body that are causing too much tension. If I was to ask you if exercise is good for you, you would say yes. But if you aren't used to exercising and you exercised for three hours, would you feel better the next day or worse the next day? You would feel awful! You feel worse the next day not because exercise is bad for you, but because you overdid it. The fact that you are using your muscles and the lactic acid has to get out through the body through the garbage system of the body. Now if you create too much garbage in your body, (too much lactic acid) it can't get out very fast and so it stays in the muscle and that's what makes your muscles sore when you exercise too much. That's what will make it sore and so if you go to your personal trainer and say "I am too sore to exercise today," he is going to tell you to get down there and start exercising to get rid of the lactic acid in your body. The same type of lactic acid will appear in a deep tissue massage and some people are in such poor shape that it could appear when they just have a light Swedish massage. You create too much lactic acid for the body to get rid of and so some people will say they feel worse after a massage than they did before. If that's the case, then they were in such bad shape that if they really needed this massage, they may need more massage to get rid of the lactic acid. Deep tissue massage can cause a buildup of this lactic acid and so therefore you can be more sore the next day than you were before, but it doesn't mean that it didn't do you any good or that it did you harm.