I made about $200 per day and had 500 clicks per day from AdSense, steady for years. Suddenly it dropped to $30 per day and 1-5 clicks overnight. Traffic was consistent, about a million daily page views, and 40k active registered users a day. 1-5 clicks was clearly wrong. I contacted AdSense support and asked if I should try making changes to the ads, or wait for them to debug the issue. They said to wait.

Well, I waited a month as they escalated it to different departments, and someone went on vacation. Finally I get an email with their official answer. It says they can't tell me the issue, because it's classified information. However, I could try labeling the ads, "Sponsored Advertisements".

That's the great support I get after making them about 100k in commission. Thanks Google.