I'm sure you have walked into a gym and seen a guy lifting weights with awful form, but that's just one of many mistakes that are made! Here are some others... Do NOT make these!
1) Going to the gym without a plan
This is one of the most common mistakes made in the gym. I lost count a long time ago with the amount of people I have seen in gyms walking around like a zombie not knowing what exercise to do next. You can always tell who has planned a session and who has no idea what to do. As the old saying goes 'failing to plan is planning to fail'.
2) Performing cardio before your lifting session
If your goal is to 'get big' and pack on the muscle, you still need a little bit of cardio, but not too much and not before you lift. Think about it, if you do a 30 minute cardio session, and then go and start your weights, you're not going to be 'fresh' for the workout are you? which could lead to poor technique, or even injury.
3) Not warming up
You get told as a young kid to warm up properly before playing sport or something similar, but many people still don't warm up. In order to prevent injury and work out effectively, you must warm up, otherwise its like trying to stretch a frozen elastic band - snap!
4) Neglecting certain body parts
The most common body part to neglect is the legs. There are some comical pictures of men with huge upper bodies but skinny small legs, but it does happen! I have personally seen that in the gym, guys who wear tracksuit bottoms and a tank top, they look great, but as soon as they take the bottoms off, its rather amusing!! You need to balance your training and make sure you train all body parts. You can focus on a few more than others, but you need to train all of them.
5) Always training the same
If you're someone that never changes their routine, reps, sets, equipment etc, then you need too! you're not going to see results by doing the same thing every workout. You need to keep challenging your body, keeping it on its toes, that way you will get the results you want.