Most common ones:

1) Jumping from program to program (or not using any program at all). The worst program done consistently, will take you lots further then the best program done well. Give a program time to work!

2) Lots of folks see working out as these "3 month get ripped summer cycles." You will never make significant progress if you just keep hopping to the gym for a quarter of the year. Make fitness a habit, you're stuck with your body for life so treat it well.

3) Spending time on a lot of accessory exercises (ones that work individual muscle groups as opposed to several at once). It's fine to do them, but try to make compound exercises the real meat of your regimen.

4) People who say, "But my metabolism is so fast, I just can't gain weight." Vast majority of the time, this is simply untrue. Sure, you can gulp down a whole pizza in a sitting, but that doesn't mean much. What about the 2 days before that when you ate below 1500 cals? If you want to put on some mass, track your meals so you know what's your calorie input for the day.

5) The ol' dirty bulk. Pretty much, eat everything in front of you. You will put on weight, but most of it's going to be fat and you'll look the worse for it. Don't be afraid to have a couple extra slices of pizza, but don't eat the whole thing, then drink some beers, then eat some wings etc.

Last thing, and this is just a little bit of a tip. Enjoy your time working out! If you want bigger arms, don't be afraid to throw in some extra sets of arms because you don't want to be "that guy" who curls like crazy. Have fun, and the gym will become like home after a while.