Dudley Do-right (http://en.wikipedia.org/wiki/File:Dudley_Do-Right.jpg)
They're trying to help us, but sometimes we don't help them.
My theory is that there are people who try to save the advertiser the cost of an ad click by copying the Url displayed in ads and pasting it into the address bar of their browser. In the interest of full disclosure, I fall into this group at times too.
Google allows the Destination Url to be different to the Display Url. This is handy when the Destination Url is complex or contains tracking parameters. Keeping the presentation simple is best for everyone.
Those managing AdWords campaigns can use this facility for reasons beyond simplicity. Some will choose a Display Url based on how the address looks for the potential visitor or an imagined keyword advantage, without considering if this actually a displayable page. Regardless of the reason, they believe if the AdWords Destination Url is valid, their job is complete.
An example is always enlightening.
Let's say the Display Url of an ad is http://domain.com/Plumber and if clicked will take the visitor to http://domain.com. All good. However, should the Dogooder copy the Display Url and navigate directly to that page they will see some form of "page not found" message. It's a matter of conjecture what happens next and will be influenced by what options are visible on the "page not found". Perhaps the Dogooder goes to the home page or maybe they return to Google and try another ad.
The Display Url must show a real page for all the Dogooders. This can be as simple as creating a new page to match the Display Url or triggering a redirect to an existing page. The potential value of a well-crafted landing page (or Destination Url) is not to be ignored (and maybe I'll discuss it further in another post).
Once you've got a visitor to your site, by whatever means, the last thing you want is for them to see "page not found" and go elsewhere.
Where's that page gone? (midboh.com.au)
AdWords fiasco to avoid - pointing to a non-existent page (blog.midboh.com.au)
AdWords fiasco update (blog.midboh.com.au)