If you are training exactly the same way, everyday, you are going to get hurt.
Yesterday, I had a few clients come up to me complaining about shoulder pain and/or knee pain. Now this was in my group fitness class and I only see these clients once per week. I of course cautioned them to always listen to their body and if something hurts, to not push through any pain but to stop immediately and if the pain persists, to see a doctor.
I know that many of you go to a number of classes per week, which is fine, but if you are doing squats in every single class, you are at risk for injuring your knee. If you are doing military press every single day, you will injure your shoulder.
You must leave a day in between training muscle groups, to allow for rest and recovery and growth. If you are not affording your body the chance to heal, than you are overusing the joint and may injure yourself. If you attend my classes everyday, than I make sure that what we do on Monday is different that what we do on Tuesday etc.
There are many teachers and many types of classes. It would be impossible for all teachers, in our club for example, to know what the others are doing and to co-ordinate the muscle groups being worked. Therefore be proactive in your approach and if you see that the teacher has a muscle group planned that you did yesterday, ask for an alternative or use lighter weight or no weight or do another exercise altogether.
I hate hearing that people are developing injuries. It is part of living an active lifestyle but it can be avoided. You have to train properly. Vary your weekly workouts. Do not just choose a time slot and insist on only attending that class because of the time, no matter what the workout is. Try a yoga class or a pilates class or a dance class. This will help you to achieve balance and avoid injury.