Because users have different operating systems, web browsers and computer hardware (including mobile devices and different screen sizes), online ads may appear to users differently from how the advertiser intended, or the ads may not display properly at all. A 2012 comScore study revealed that, on average, 31% of ads were not "in-view" when rendered, meaning they never had an opportunity to be seen. Rich media ads create even greater compatibility problems, as some developers may use competing (and exclusive) software to render the ads (see e.g. Comparison of HTML 5 and Flash).
Furthermore, advertisers may encounter legal problems if legally required information doesn't actually display to users, even if that failure is due to technological heterogeneity.i In the United States, the FTC has released a set of guidelines indicating that it's the advertisers' responsibility to ensure the ads display any required disclosures or disclaimers, irrespective of the users' technology.