There are some important SEO mistakes

1. Unethical SEO Consultants :
Nowadays, u can see many Companies providing SEO services. Some SEO guys use purely ethical SEO techniques, while others use unethical SEO. Even the person who hired the SEO guy do not know about this. When his site get punished that time he came to know.
So the conclusion is, if you want to hire SEO, choose a reputable SEO consultant, one who will keep in regular contact with progess reports and updates.

2.Use of black-hat techniques :
Same as i mentions above, techniques such as doorway pages, stuffing keywords, hidden text etc are the examples of black-hat tech. So if you want long term success, avoid such techniques.

3.Expecting Immediate Results :
you wil never get immediate results in SEO. You must have patience in you. Succesful SEO requires good link building which takes time. SO you must agree to spend lots of time with good patience.

4.In-house optimization :
In past even now also many people think, they can do SEo their own. They feel SEO is an easy task to do.
But SEO is not that much easy. It involves undrstanding google's complicated algo. and according to that they have to optimize the site to rank well in SEs.

5.Untargeted keywords :
Many times some SEO ignore main keywords and only work for untargeted keywords. After getting good rankings, if they dont get enough traffic, they complain about that.
So they have to choose right keywords for optimization which give them good traffic.