From watching members train at my facility, and countless gyms throughout my 25 years of training, there are some mistakes that I see over and over and over again. These mistakes can put the individual at risk for injury to the body and may give them sub-par results for their time invested.  So,  here are the two big mistakes I see occurring repeatedly. 

1) Poor Bio-Mechanics:
Bio mechanics is the study of the action of external and internal forces on the living body, esp. on the skeletal system. Basically, poor form not performing the particular exercise correctly.  This usually occurs because the individual is using to much weight and must cheat to move the weight to complete the desired repetitions.  By using less weight and tightening up the form the particular muscle will be overloaded more efficiently thus producing more hypertrophy (growth). 
There are times when cheating is a legitimate technique to be used.  But usually only advanced trainers understand the application. It should always be used to make an exercise more difficult not easier.

2) Not Connecting the Mind to the Muscle
The second mistake I see on a daily basis is not connecting the mind to the muscle being trained.  First let me give you an example that probably occurs in your life. You sit down to read a book or magazine and you're not particularly interested in the material you are reading.  After reading for a half hour you put the material down. Later in the day you realize you remembered almost nothing that you read.  Because you weren't engaged in the subject matter the information did not stick. The same holds true for lifting weights. 

You can go through the motions but unless your concentrating on the individual muscles being worked - by focusing and really using the proper amount of resistance and repetition speed the results will not stick.  A good example of concentration is squeezing the muscle at the point of contraction (called peak contraction). This helps maximize your time.

If you went grocery shopping without a list, most likely you would end up buying items you didn't need, and you would forget to purchase items you had intended to get.  So by taking five minutes and writing out a list you ensure you get everything you need and nothing you don't.  The same holds true in the gym take five minutes and write out your routine for the day.  Exercises, sets, reps, rest time between sets. This way you have a game plan and it will guide you and motivate you to complete your agenda. 

So to sum everything up:  Make a plan and follow it.  Focus all your mental energy on the task at hand. Use proper technique/form on ever rep of every exercise and reap the rewards that come from your time invested from training.  And when you're not sure how to perform an exercise ask a member or employee of the facility to show you the correct form. That way it's a win win for member and owner because now you're seeing the results that you desire. And this alone will motivate you to want to train.  Nothing begets success like success!