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To test it Nick bookmark the page you found, delete your cache and see if they are the same ads.
The ads should be relevant to the page, and if that comes up in a SERP should therefore be relevant to the search.
ADD in it would be best to test it on a page that has been targetted rather than a 404. Putting ads on a 404 even for test purposes is not a good idea imho.
There have been complaints on the boards here about pages with content that change frequently, because the bot does not come back on a regular basis to recheck the page.
<edit>clarity, bad sentence!</edit>
[edited by: Jenstar at 7:36 am (utc) on Sep. 11, 2003]