In 2013 Google removed as many as 350 million ‘bad’ ads from AdWords in order to prevent abuse from both advertisers and Adsense publishers. Google didn’t publish in much detail why any ad in particular was removed; it seems the most common reasons ads tend to get removed are for products that violate AdWords terms, ad copy issues (e.g. using all caps or multiple punctuation marks) or having a display URL that doesn’t match the destination URL (in the case of affiliate ads).

Google have released an interesting infographic that will help advertisers stop getting ads disapproved in the future.