There may be agencies out there that have their own example of a target that's too narrowly defined. A target of one-armed bird watchers who live in green houses, or the club-footed home office radiologist with an added demo filter, leaves an audience of only a handful of people.

Behavioral targeting providers have seen their partners overreach with their targeting desires. One can't fault zealousness in trying to do anything you can to find your audience and target them.

One such example I came across was an electronics company that wanted to market its LCD TV to "early adopters." A Revenue Science newspaper publisher decided to build a segment based on multiple recent views in the technology reviews section. However, they weren't able to get the reach they were looking for because they had defined the segment as "a person who read four pages in the technology reviews section in the last two weeks." 

If you are a site with an audience the size of Yahoo! or a vast advertising network, perhaps slicing your targeting criteria so thin can work. But most sites aren't big enough for the kind of atomic segmentation that the electronics company wanted.

In order for the advertiser to get the reach they needed to deliver the campaign and ultimately make it more effective, Revenue Science suggested they add another target segment phrase to include search key words and articles with specific target words that implied interest in the latest television technology; for example, 'flat-panel,' 'DLP' and 'HDTV,' and then separate the conditions with an "or" statement. This generated a larger audience of people who still demonstrated that they have specific interest in the latest television technology.