When it comes to hitting the gym, what you should do can be summed up relatively easily: Nail your exercise form, train smart, and apply maximum focus to every workout. But the list of what you shouldn't do is considerably longer. Coaching cues like "Don't round your back" or "Don't lock out your arms" are common advice from good trainers, but there's plenty of wisdom that isn't tied to specific movements. With that in mind, here are eight additional things you should never do at the gym if you want to supercharge your results! 
1

Never rest on your can during a leg workout 

You've just finished a grueling set of leg curls, and your only thought now is to sit down and rest between sets. OK, you can get away with a second or two, but parking on your butt for long rest breaks on leg day is actually counterproductive to recovery. Walking around helps flush the muscle of the byproducts (e.g., hydrogen ions) of high-intensity training that cause muscle fatigue. 
If you sit your ass down on a bench or don't bother to get up out of the machine, you're actually reducing the blood flow to your muscle. Blood helps flush metabolic byproducts, so limiting blood flow makes it take longer to get ready for your next set. In other words, if you sit around between all your leg exercises, you'll simply be "less recovered." 
Catch your breath, but try to keep moving on leg day for better recovery! 
Catch your breath, but try to keep moving.
2

Never blindly copy an exercise someone else is doing

You know how somebody catches a virus, and pretty soon everybody's sick? That can happen with exercise technique gone bad, too. One guy does something wrong, and sooner or later, you see everyone doing it with the same bad form. I've found this to be especially true with rotator-cuff warm-ups and the Romanian deadlift, which many lifters turn into a sloppy pseudo-deadlift. 
Just because the biggest guy at your gym does an exercise doesn't mean he's doing it correctly. Observation is a good place to start when learning new exercises, but it's just that—a start. To really nail the finer points of tough exercises, check out the Bodybuilding.com Exercise Database, where you'll find written and video instructions for hundreds of moves. Working with a knowledgeable coach or training partner can also help. 
3

Never use a balance board during a muscle-building workout

Balance boards were popular among trainers about a decade ago. Balancing yourself and doing an exercise movement simultaneously isharder than lifting with your feet planted, but if your goal is to build strength or muscle, the trade-off isn't a good one. You simply have to sacrifice too much weight when training on an unstable surface, and you won't get the same returns if your primary goal is to build muscle size. If you want to train for stability and size, you're better off training for each separately.