I would email them myself though just to be sure because it does look like they might be contradicting themselves. Plus if you do get banned you can point to the email.
Adsense terms and conditions still prohibit using Ads on error pages See 5.Prohibited Uses Maybe the ads will be ok, if the error page contains some content; however a non-content error page with just ads would definitely violate Google's policies.
Wow you kinda suck. Say someone has Potatoe selling site and uses adwords to markey their site. How the hell would they get good traffic from a friggin 404 page. Really really stupid
you can't ,if you read correctly when you were registereing at adsense,they say: do not publish the ads on "under construction" sites.
Will the Adsense bot even bother to parse the 404 page? If not, then you're not going to get any relevant keywords. If if it did, keywords relevance is still going to be a problem. Not worth the effort, I'd say. Cryo.