This is actually an appeal - to the experienced users out there - what the the common mistakes that newbies make? how to spot them, and how to fix 'em.
Obviously I'm perfect, never made a mistake in my life, so can't contribute! 
1.Not working the muscle,I can't stress this enough when you are doing your set your mind should be focusing on that muscle squeezing and contracting the whole way through.
2.Ego lifting-when you hit the gym tune out everyone respectfully,You have nothing to prove to anyone.
3.Staying Consistent-Set realistic goals like 1,2,3 year goals because we all like to think we will have our dream bodies in 3 months LOL which invariably leads to disappointment.... it is a lifelong commitment so enjoy the ride =)
4.Doing the same routine day in and day out-MIX IT UP use different rep schemes or techniques such as rest pause,drop sets,giant sets.total failure etc etc rotate them into your routine have fun
Not writing sets and execises downn there is know way you can remember from week to week and counting macros and calories there are too many apps and smart phones are smart for a reason
Trip hit you with some good ones. I hate to see a beginner not focusing on the lift. This is a mind game as well. I try to help people understand slow controlled reps with squeezing at the end of the positive rep and slow negatives. As far as AAS use. Some guys want to lift for 2 weeks and start a cycle. Beginners need to use low weight and CONTROL their lifts. Most beginners are very sloppy..
If you feel that last rep is probably going to hurt you (especially on deadlifts and squats), don't do it.. put the weight down. I've been out months on end because of trying to squeeze out on exercises that require perfect form and ended up injuring m yself. I am all for half reps, forced reps and going all out especially on the last set, but there are certain body parts that require good form (all the way).
Never sacrifice form for weight.
If you're natty don't compare yourself to enhanced lifters.
Don't feel ashamed to ask experienced lifters questions.
Keep logs to track your progress.
Set short, mid, and long term goals.
Listen to your body. If it hurts don't keep doing it because the eventual result will be injury which sets you back.
Never EVER EVER EVER skip leg day!!! Nothing worse than the hulks body on Urkels legs...
talking bout those guys with arms bigger that legs like there gonna fall over nah never seen one