Sounds like a javascript caching problem. Have you tried clearing your browser cache?

I sometimes get this weird problem in Safari on the mac, after I change ad placements, Safari shows one ad where the other should be. Scared me at first, but closing and opening Safari makes it go away, proving it's something on my end, and visitors are seeing the ads OK.

Sounds like you have something similar.

First i remove all javascript and cache, and problem is there. it happens when open the url for the first time, if user do a refresh block appear well. First i suspect browser cache problem, but it isn't i test in another computers and in other places , problem is there.
This issue happen in IE only. I use google coop, turn it off, i remove the other blocks nothing make changes in this. Impressions count, but CTR is lower then normal, this problem start in this intermitent way at Oct 14, and make my worst days in adsense.
Site have more then 50000 impressions daily.
I'm crazy with this!

Do you have any other javascript on the page besides the adsense javascript?

yes another javascript is in page but i turned it off and problem is there.

I solve this issue, removing ad code and create a new one with same size etc etc, the problem solved, but why this happen before. The only diference is /* bla bla bla */

Site is non-english and in /* bla bla bla */ i have a special Char, at first look i think the problem is here, but, i have many channels without problems with this special non-english charact, for sure i don't know what's up with this ad code, but is solved.

A similar thing is happening to me. Site is in english, happens both in IE and in Firefox. Ads don't show on the first time, but random. Sometime they do. On refresh, they show.

On firefox, the dev toolbar does not show any javascript errors, it only shows warnings, and in the usual case it used to show warnings with google's code.

Seems like its now working... did nothing, so I suspect it was a problem from their side.

I'm getting this too (Mac Firefox).

Suddenly my ads are failing to display in IE but work fine in Firefox. I just get a white block and the status line shows that the ad is loading but never does. I haven't changed anything and I've got the problem on all my sites. What's happening?

Beggers: can you see Adsense on other sites that are not your sites? That's always the first thing to look at. It will tell you whether it's something in your browser (can't see Adsense any site anywhere) or something on your pages.

I checked my sites when I got home and the ads were working again. However, they stopped for about 20 minutes and then came back. So far things are back to normal so it must have been a glitch at Google's end. I didn't think of checking other sites when my ads were down -- that's a good tip and I'll remember that next time. Thanks.