Thousands of advertisers are defrauded every day. Are you one of them? If you trust an ad-network to display your advertisements then you probably are—here's why:

Hackers Hijack Millions of Computers in 'Massive' Fraud Case
"The U.S. charged seven people with a 'massive' computer intrusion scheme that used malicious software to manipulate online advertising, diverted users to rogue servers and infected more than 4 million computers in more than 100 countries...[they] made millions of dollars by manipulating the Internet searches of infected computers, redirecting users to sites they never intended to visit or swapping out advertisements on web pages, according to the indictment... Manhattan U.S. Attorney Preet Bharara says 'We believe this criminal case is the first of its kind'"...You're wrong Mr. U.S. Attorney...it's been going on for years! Advertisers pay for fraudulent clicks every day!
(Nov 9, 2011)—Bloomberg Article

When you trust an ad-network to display your advertisements:

1. Placement: You may think you know where your ads are displayed, and sometimes you do, but many ads are displayed at the bottom of pages where they're never seen, or next to revolting, racist, unmoderated comments. Message boards may be an inexpensive place to have your ad displayed, but you always run a risk that the ad may be placed next to revolting comments. If you're concerned about your brand, avoid message boards. The ad-network has no control over the content on a message board.
2. Clicks: If your ad is displayed you have no idea who clicked it because the ad-network can't tell you who clicked it. Log files provide IP addresses but an IP address can be spoofed. You're probably paying for clicks from click-groups and bots that run through trojan proxy servers.
3. Publishers: Many ad-networks don't tell you which publishers display your ad; it could be displayed on splogs (spam blogs), within invisible I-frames, underneath stacked ads, at the bottom of pages which are never seen, in spam mail, or on pages that cycle ads. Some ad-networks let you choose the sites, or type of sites, that display your ads. If you specify which sites are allowed to display your ads then the odds of being a victim of fraud is less likely.
4. Ad-Blockers: Your advertisement could be blocked or hidden by ad-blockers. Text ads (not including Google Adwords® ads) are more likely blocked and counted as being displayed than Graphic/Flash ads, which are usually blocked but not counted as being displayed.