From an agency point of view its difficult to monitor if a client doesn't use proper 404's for broken pages. I've seen this so many times its ridiculous.
Adwords will pause ads if the Google Adwords Spider encounters a 404, if it doesn't it won't be able to tell the difference.
Setting an end date to a campaign is a no-brainer though.
I wish these people with money to burn would send some my way though
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