So let's get started with the following mistakes:
1- Doing uncomplete movements:
Some exercises make you much efforts and can be difficult to complete the whole movements, for example, when you left weight to build your chest don't make the move until the middle, make it whole down until you touch you chest, because, if you do uncomplete move you may block chest growing and still like a chicken chest.
2- Focus on the same piece of you body and getting sooo tired:
don't just focus on chest as an example, but follow your trainer Instructions use your power not Momentum.
3- Take a long relax:
when you are fitness or doing great left weight don't take a looong time to relax, you have one hour of exercises, right? Keep this hour by word DONE, when to the long relax your muscles get cold back, you have to do warm up workout again, so one hour will be two, then you wasting time doing nothing benefits your muscles and more loosing energy, -I was doing a long relax before this article-. Time of relaxation not more than 2 to 3 minutes.
If you don't know what you do, always ask and follow your coach or trainer instructions, is the main reason that I missed.