Indeed the question is quite general.. but I'll try to answer with general problems as well.
Muscle imbalance can be a problem if you train incorrectly. For example, if you focus a lot on push-type movements (pecs, triceps, front delts), as would be specific in a striking sport, but don't work pull-type movements as well (back, biceps, rear delts), this will create a strength imbalance.
Also similar is training the abs very hard if the lower back is ignored, that can also lead to some serious problems.
I think not training your core well enough can be a problem as well. As you get stronger, bigger and faster you want to make sure that your body can keep up.