Did you know that the Google AdSense competitive ad filter has a max limit of 200 URLs? I didn't know that. But a WebmasterWorld thread has discussion on what steps to take when you reach the 200 limit.
Some say you should export your list and manually go through each URL to see which URLs are now inactive. Remove those from the list and then you gave yourself some more room to work with.
I figure it may be easy enough to create a small program that does a check for you weekly, monthly or daily of your filter list and notifies you which URLs go dead.
Hmm, wouldn't it be nice if you can automate that directly in Google AdSense or an API?
Forum discussion at WebmasterWorld.