A featured WebmasterWorld thread has a fun list of "not to do with your AdSense account because Google might ban you for these things." Here is the list complied by a new member, BrandNewDay:
- Click your own ads
- Asking other people to click your ads
- Joining a program that promises you that people will click your ads if you click their ads
- Telling all your friends about your sites
- Exchanging Adsense code with other people or programs
- Buying and/or operating so called "Adsense ready sites"
- Having sites with only ads and/or search results on them
- Taking/stealing other peoples content without their permission
Of course, with any list like this, there are arguments and discussion about the details.
Later on, WebmasterWorld moderator, httpwebwitch, adds some more:
- allow malware to be installed on your server, or on the machine which is used to access the account
- allow multiple people to log in to the account using the same password
- log in through an anonymizing proxy
- hack into the system and start requesting Adsense ads in raw XML
- mess around with the iframe in which they're displayed
- try to sidestep the cross-domain scripting limitations using a server proxy
The list can go on forever. One thing you should not do, a huge AdSense No-No, in my opinion, is obsess over each rule.
Forum discussion at WebmasterWorld.