These are things you never want to do at the gym if you can help it.
Not Having a Plan
People often come into the gym and just begin doing whatever is convenient for them, whatever they feel like doing at the time, or whatever machine is open at the time. This is foolish because you will end up neglecting important areas of your body and overall won't get a good total body workout over the course of a week. Muscle imbalances will creep up on you and that will hurt you in everyday activities as one part of your body will be strong while another might be extremely weak. Your body needs to work in synergy with each part doing what it needs to do efficiently or it won't work very well. You don't need to write every single exercise you do into a journal or anything but you should at least have a good idea of what type of workout you want to do before heading to the gym.
Not Leaving Enough Time to Finish Your Workout
Many people head into the gym a little later than they want to and try to bust out a bunch of exercises in a short span of about 30 minutes or so. But they don't alter their workouts accordingly and end up not getting all the right exercises in for certain body parts. That will come back to haunt you in the future.
Not Trying Hard Enough on Cardio Machines
It seems that no one puts enough effort into cardio at the gym because they don't want to sweat or look like they're trying all that hard. Many people simply curl up in front of a magazine and ride a stationary bike for about 30 minutes without realizing they haven't even burned a measly 100 calories in their entire workout session. Make sure you tell yourself when you head to the gym that it is a time to work and get the maximum results because you're cheating yourself if you only go in there to look good.
Using Machines Instead of Free Weights
Machines are good supplementary pieces of equipment for targeting certain body parts after you've built a strong base, but other than that you should use free weights whenever possible. Free weights will tone your body up and down and give you more strength in your secondary muscles to use in everyday activities. Machines make you more injury-prone due to imbalances you will suffer by not working stabilizer muscles.
Resting Too Little Between Sets
If you're working out to gain strength then you need to give yourself a good 45 seconds to a minute or so to rest (and even more if you're maxing out) because you need to give your body time to get back near full strength. This will give you the ability to lift more weight thus challenging your muscles more and making you stronger. Don't worry about looking like a slacker in front of other gym goers because it really doesn't matter. You're there to get work done not to look good.
Good points--I particularly like that you address muscle balance. I think failure to keep your muscle groups relatively balanced is a good way to get hurt.
One important point I do think is missing is form/technique. Particularly when using free weights, people have a bad tendency to sacrifice form to get that additional 5 pounds on the lift.
One important point I do think is missing is form/technique. Particularly when using free weights, people have a bad tendency to sacrifice form to get that additional 5 pounds on the lift.