1. Clicking ads on your site. This is the biggest of all the adsense mistakes and an easiest way to get your account banned.

2. Editing adsense codes. You are not allowed to edit the adsense codes you obtain from your adsense account. It is also advised to make the color changes etc from the adsense account itself instead of editing it from the codes. This is to avoid the possibility of mistakes.

3. Placing more ad units than allowed on a page. Only 3 ad units, 1 link unit, 2 search tabs, 4 different referral buttons are allowed on a single page. It should always be made sure that all your pages are within the allowed list.

4. Using other contextual PPC ads together with google ads. You are not allowed to display other contextual ads on a page which is showing google ads. Google ads, YPN ads etc are examples of contextual ads. Amazon ads are examples of non contextual ads. However there is a debate on whether amazon's omakase is contextual or not.It has been informed by adsense support that Amazon ads can be displayed on the same page with google ads.

5. Labeling ads with texts other than "sponsored links" or "advertisements". Only the above mentioned two labels are allowed for google ads. I have seen that some publishers uses labels like "sponsors" "links" "links of interest" "click here for more info" etc. I can assure that these are against adsense TOS and can get your account banned.

6. Disclosing your adsense reports data to public. You are not allowed to disclose anything other than your aggregate earnings to others. I have seen many people copying data from their reports and pasting with their questions on different forums. Everyone must know that this isn't allowed at all.

7. Placing direct download links for music or video on your adsense site. According to adsense TOS, no link on your site should directly lead to audio or video result.

8. Too many entirely different topics on a single site. This mistake won't get you banned but this can make it difficult for you to get relevant ads.

9. Not updating your site often enough. This also won't get you banned but will affect your earnings and traffic. A site which is updated regularly will be getting a lot of return traffic and the ads will be changing from time to time thus the chances of same ads appearing for too long and people becoming not interested in clicking on the ads is reduced.

10. Not blending ads with your pages. To get a good click through you should make ads look like a part of your site and not as an alien from mars.

11. Setting up the payment hold for too long. Adsense is very strict about click fraud and they can ban any account if they detects fraud clicks.Once you are banned they won't pay you the money you earned before you was banned. So it is not a good practice to set up a payment hold for too long a time interval.

12. Not checking your adsense reports at least once a day. If you don't check your reports regularly you won't know when something suspicious happens on your account. If you are checking it regularly you can write to adsense support whenever you find something suspicious and it can save your account from getting banned.

13. Not reading adsense and PP carefully.
If you haven't read it yet go read it now. Its a must for every adsense publisher.