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.