No

Hobbs has summarized this pretty accurate. Adsense on error pages is unkewl. ;-)

Noop, nothing is allowed on error pages, even search box is not allowed. Read TOS.

I serve much over 10k forbidden 403 pages daily, half of which to bots, and that's not counting the 404 not found pages..
Imagine how much an almost guaranteed 5k daily ad clicks could make you even at 1 cent a click, an extra $1,500/m yet Value to advertisers = Zero

If you have no respect for your visitors and no plans for turning them around to other pages or sites, Google "monetize 404 pages"

Actually, Google removed these prohibitions when they updated the TOS in February - reference Jensense posts in February archives:

Removed - what's in bold italic in this paragraph:

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;

I don't put ads on my 404 pages, but I don't think there's anything morally reprehensible about a search box.

in any email

Pardon?

AdSense in emails are allowed now?

Can we have this clarified?

I'm just reportin' what I found. I'd personally wouldn't advise trying it without a confirmation from Google in writing, with a gold frame and certified by a notary public.

Removed - what's in bold italic in this paragraph

That calls for clarification by [drumroll]ASA[/drumroll], doesn't it?

OTOH, Hobbs is probably right that the value to advertisers is, erm, probably rather low. Also there might be problems with targeting (which lead to junk ads, which leads to low EPC). Then again, where's the difference to a parked page?

In the Adsense Programs and policies, it says

No Google ad or Google search box may be displayed in a pop-up, pop-under, or in an email.

I dont see anything about 404 pages

i mailed adsense support regarding this issue, but they didn't replied properly. they simply said, check the adsense groups :-)

Why even bother to try and push the boundaries? Do your viewers a favour and forget it.

It would be nice to be able to put an ad in a Newsletter though.

I send out a daily newsletter and I agree, it would be nice, although I understand why Adsense prohibits it. Can you imagine what would happen if every one of the millions of spams sent around the world every hour also included an Adsense ad?