You shall not, and shall not authorize or encourage any third party to:...
(v) display any Ad(s), Link(s), or Referral Button(s) on any error page, on any registration or "thank you" page (e.g., a page that thanks a user after he/she has registered with the applicable Web site), on any chat page, in any email, or on any Web page or any Web site that contains any pornographic, hate-related, violent, or illegal content;
So if you have a nice custom 404 page or a nice thank you page or confirmation page or registration page, according to this, you cannot place Google AdSense on them.
But we linked back to an older thread where AdSenseAdvisor said:
As mentioned in the blog post, our policies were designed to prohibited the placement of ads on non-content pages, so you'll need to use your discretion to determine on which types of exit pages it would be appropriate to display Google ads.
For example, if "Thank you" is the only text-based content on the page, AdSense technology would not be able to serve relevant ads. However, if your sales confirmation pages also contain other text about your business or product, AdSense can provide a relevant 'next step'.
So it appears, that the above prohibited use only applies when that error page, thank you page or registration page has little to no content for AdSense to match on. But that doesn't make complete sense to me. Why would the AdSense TOS say it is not allowed and then AdSenseAdvisor say it is OK in these cases? I guess they turn their head towards certain implementations, which I think is perfectly fine.
Forum discussion at WebmasterWorld.