Fraud & invalid traffic · also called Unintentional click, Fat-finger click
Accidental click
An accidental click is a click on an ad that the user did not mean to make, usually because of where or how the ad was placed.
Not every worthless click is fraud. On a small phone screen a thumb aimed at "next" can land on an ad. A page that shifts as it loads can slide an ad under the finger. A link styled like content can be tapped by mistake.
Google names accidental clicks caused by poor ad placement as its first example of invalid clicks, and filters them where it can. Its Product-Integrated Feature policies forbid placing search units in ways that encourage accidental clicks.
For arbitrageurs this is a design discipline. A lander that puts related search terms right beside navigation buttons, or that loads the unit late so the layout jumps, will show a flattering Lander CTR. But the people who arrive on the results page by mistake leave at once, advertisers see no value, and the feed's quality signals fall. A deliberate layout built to cause mistakes crosses into implied functionality and is treated as a policy violation.
The healthy sign is a click rate that is good but believable, with visitors who then engage with the advertiser.
An example
For example, a lander has a sticky "Continue reading" bar. Half a second after load, a related-search block slides into the same spot. Many taps meant for the bar hit the block. Those are accidental clicks.
Related terms
Invalid click
An invalid click is a click on an ad that does not reflect genuine interest in the advertiser, so the advertiser should not be charged for it.
Implied functionality
Implied functionality means an ad or page element pretends to do something, such as play a video or download a file, when a click really opens ads.
Lander CTR (keyword click rate)
Lander CTR is the share of landing-page visitors who click a keyword or related search term, taking them on to the page that shows search ads.
Ad disclosure
Ad disclosure is the clear labelling of paid content as advertising, so people know when a link or listing is an ad before they click.
Policy violation
A policy violation is any breach of the rules set by a search feed or traffic source, from a misleading creative to non-compliant traffic or a wrongly declared parameter.
Sources: Google Ads Help: Invalid clicks, definition, Google AdSense Help: AdSense for Search Product-Integrated Feature policies