It really isn't cool when you're busy entering tons of data for Google AdWords and your site goes down. Consequently, Google cannot verify the site and cannot check your landing pages and your quality score probably is not where it could be. That's what happened to a WebmasterWorld members, who writes:
So I check the URL during the ad creation process to make sure it is up and it is but by the time I submit it the URL is down. So since the landing page could not be accessed all the quality scores on the keywords are terrible causing high minimum bids ($5 - $10).
While this is a bit worrisome, I'm sincerely hoping that the guy's website does not go down when people are actively surfing. It wouldn't be cool either if you're paying for clicks and your site is down when the visitors are clicking on ads. I'd think, first and foremost, that focusing on a solid uptime (and decent host) is probably a better investment.
But in any event, a forum member suggest that it may be a good idea to change your landing page destination URL first. If that fails, change the domain name. According to the user, "My understanding is the QS will get updated if the destination URL changes, but am not sure if you have to change the domain too."
If you have any additional feedback and recommendations for the advertiser, you can contribute to the discussion at WebmasterWorld.