Publishers are faced with a potential revenue issue as Google's DoubleClick ad server is currently down.

While websites usually present an ad before the main homepage comes up, all that appears right now is a text in a blank space that reads "waiting for ad.doubleclick.com," which is causing some pages to load much slower than usual, according to Business Insider. The issue affects several sites, including Business Insider, Forbes, The Wall Street Journal, BBC.com and Mashable.

The malfunctioning server could have a severe impact on the sites' advertising revenue, and could cost them thousands, maybe even millions, of dollars.

This incident is not the first time that Google's DoubleClick for Publishers has gone down, the previous being in March 2013, when it went down for several hours with DoubleClick for Advertisers and Dart Sales Manager, Business Insider reported.

The problem has caused several webmasters to voice their displeasure on Twitter, and the search giant is currently working to fix the issue. Google has yet to offer a comment.

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