reillybrown:

That’s a tough question to answer, do you mean the biggest mistakes or the most common mistakes?  For people breaking into the industry, or those who have been long established? 
Maybe I’ll throw in a little of all of that.

The #1 mistake I see from young artists still trying to get their feet off the ground is a lack of backgrounds.  Heh, I swear to god, every single FAQ about what should go into a portfolio stresses how important it is to have finished pages with finished backgrounds that show off your grasp on environments and perspective, but when I look at artist’s portfolios it is RARE to see anyone actually doing that.
Then, if you do manage to do some backgrounds, make sure you use a ruler to keep the lines straight, and the perspective on point. 
Also, whether you’re drawing perspective, or not, use some reference to get a sense of what a building, or a mountain, or a tree actually looks like.  No lollipop-trees like you’re in 1st grade!
If you can manage that, I guarantee you’ll be head and shoulders above the competition.

The second mistake I see a lot, again, especially in younger artists, but even in pros sometimes, is too much fan art.  There are two reasons this is a problem— first, character design is an important artistic skill when it comes to drawing comics, and that skill needs to be developed.  If you’re just drawing designs created by other people, you’re missing a chance to hone your own unique sense of character design, and learn what it takes to develop your own characters.
The other problem with this is that if you’re just drawing other people’s characters, you can’t do anything with them, because you don’t own them.  If you come up with your own character, let’s say Captain Shark-Face for instance, you can make your own story about him, publish it on your own or anywhere else you have the opportunity, put him on a tee-shirt, make an action figure… and no one can stop you!  If you’re just another artist drawing unsolicited pictures of Spider-Man, all you can do with it is post it on Deviant Art or Tumblr.  You’re sort of tying your own hands.

The number three mistake is lack of communication.  This is especially true when you’re working with a creative team, because as any high school coach will tell you, communication is the key to teamwork.  Far too often an artist gets a script form a writer, and that’s all that ever passes between them.  The artist will say, “this part doesn’t make any sense to me, but I guess it’s what he wants…”  That’s not exactly teamwork.  And you don’t know how many times I’ve heard of an inker not even getting the script!
It’s important that everyone’s on the same page. 
If the writer and artist are different people, it’s important for them to talk, so that the writer knows what the artist’s vision for the project is, and how to fit that into the plot, and the artist knows what it is that the writer’s trying to communicate— the stuff that’s not necessarily in the bare script breakdown.
If someone makes a mistake somewhere along the line, good communication can solve that, instead of staying quite and assuming that someone else will solve the problem later.
If someone’s running behind on something, and is in danger of missing a deadline, if everyone’s talking they can figure out a way to solve the problem before it becomes a problem, or at the very least plan around the scheduling delay. 
Also, more communication just makes everyone closer as a team, and makes for a better work experience.

The fourth mistake, which is a big problem for a lot of artists at every stage of their career is time management.  Making comics is hard, and if you’re serious about it, it takes up ALL of your time.  You have to learn how to balance your work responsibilities with your life responsibilities, without letting either suffer too much. 
I’ve seen people rush work to meet a deadline and make an editor happy in the short term, but the rushed work doesn’t look as good so they lose the job, or get their pay cut, or the fans simply stop being interested in their work. 
On the other side of the coin, I’ve seen people take their time making their work look perfect, and everyone loves it, but they can only do a few issues a year, and it’s hard for a series to get traction at such a slow or irregular pace.  Also, the fewer projects you do, the less income you make.
It’s a give and take, and it’s not easy to do, but you have to find the right middle ground where you’re combining speed, quality and productivity in the way that serves you and your career the best. 

The fifth mistake, I think, is something you see in a lot of artists who have been at it for a long time, and that’s stagnation.  Sometimes comic creators have been drawing the same thing for so long that it’s just not interesting to see them do it anymore.  We’ve seen it already.  What’s more, sometimes the older stuff was better, because now they’re just doing it by rote, and all the interesting problems they used to work out are solved, and they have a simplified shorthand for everything.  As an artist, it’s important to take in new influences, try new styles and new media, and find new artistic problems to solve.  That will keep you inspired, and keep your work fresh, alive, and interesting.   
Doing the same thing over and over will get as boring for everyone else as it will for you.

Great advice for comic creators!