Click bomb means sending huge number of unsolicited unusual looking clicks in order to ban some adsense account.
The intent could also be to stop a competitor dead. No income on their website hoping they can't keep up on the expenses. This is why you should have more than one type of ad.
that is not always the case. most of the time its done by sickos who didn't get enough love or attention in their life. these people's lives are so plain and eventless, they find whatever resources available out there to make it a little colorful. clickbombing someone else's adsense account is one of them.
Do you think people ever do it by mistake? Just keep clicking it because their drunk or high? I've always wondered why someone would do it. Only thing I could really think of is to cut off someones money because of envy.