Invalid Clicks: Invalid clicks are planned clicks from site owner or when the owner of the site is asking friends or visitors to click on the adds, most publisher think they are smarter than google and make use of softwares to click on their google adds, your adsens account can be disabled any time invalid clicks are discovered on your adds.
Traffic source: your traffic sources also contribute to what types of visitors that come to your sites, sites that have visitors from spam sites are not allowed in Goolgle adsense programs; if this is discovered by google your account will be deactivated.
Add positions: many publishers position their adds where people can click on it unwillingly, this is not the type of clicks google adsense want on their adds , when visitors click on adds by mistakenly they will want to select back button on their browser to get back to your site, this is not allowed and it might lead to deactivation of you google adsense account, the best way to do this is just to blend your add colour with your site colour.
Note: you are not allowed to display more than three image ads and three links adds on your site, it is recommended to put just two image adds and two links adds.

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