7 most common mistakes that violate Adsense TOS:

1.Clicking your own Ads/ Telling others to click your Ads:
This is the most common mistake done by publishers.You are not supposed to ask someone to click your Ads.Google finds out these cheaters quite easily.

2.Changing the code:
Removing some part of Adsense code or adding something to it is not allowed.Some of the publishers tried to remove "Ads by Google" from the Adsense code.As a result,they were banned from Adsense.

3.Putting the Ads in emails

4.Putting Ad blocks more than the limit:
You can not put more than 3 text(including Ad links block) and 2 search boxes.

5.Illegal labeling of Ads:
Labeling the Ads with words like "click here","help me","click and watch" etc. is against the Adsense TOS.
You can label them as "advertisements" or "sponsored links".

6.Using other programs like Bidvertiser, Adbrite:
The programs which work in similar way as that of Adsense should not be run simultaneously with Adsense Ads.
Yahoo, Chitika will work with Adsense.

7.Placing Ads on pages which do not contain any content:
Placing Ads on Pop-up windows,Login page, Welcome message is not allowed.Advertisers pay for the Ads kept on the pages containing some content.