I haven't made any mistake but there are so many people go to the gym for fitness and for being muscular, but some of them make mistakes that have a bad effect on health. If these mistakes are repeated over and over again, it can worsen rather than becoming a health.

Things one should avoid while working out:

Do not Exercise Without Trainer
 
Many people start exercising only after seeing them on TV and the Internet, which starts complaining of pain in the waist, joints, and shoulders. Be sure to consult the instructor before exercising of any kind. They will tell you according to your need, which exercises should be delayed. Do not start exercising without asking a trainer as this will not lead to the proper development of the muscles.
 
 Getting Started With Warm-Up
 
Do not start exercising on the machine directly by going to the gym because it can cause back stiffness and body pains. First of all start with a lightweight exercise. Like warm-up and stretching, this will open your body.
 
 Take Break after a while
 
Break in between 2 minutes while doing workouts. Rest for at least 1 minutes after setting 2 sets of exercise. If you exercise continuously then you will get tired soon and you will not be able to continue the exercise.
 
 Do not exercise excessively

People often exercise more and more in a hurry to lose weight. By doing this, the body starts getting tired, this can make your health worse.
 
 Do not eat immediately after exercise
 

People do not keep the difference between workouts and food at the right time and sit down to eat immediately after the gym. Do not go empty-stomach to the gym. Take a light wholesome breakfast such as apples, banana biscuits and so on.