Here is an example:
According to Adwords there is a medium high degree of advertiser competition for the phrase "visa logo"
Yet when I go to Google and do a search for "visa logo" I get zero ads.Click for larger image
So, what is going on here?
When I am working with Adsense or Adwords I have started doing searched to verify that there are competing ads in Google. In Adsense, if you are trying to write content for a high competition ad, to get highly paid clicks, then you will want to know that there really are NO advertisers for that keyword.
In Adwords, no advertisers can mean a niche that none of your competitors have discovered, which is like finding money on the street.