http://www.digitalpoint.com/tools/adsense-sandbox/ sometimes, it display Adsense and sometimes, it display DigitalPoint's own ads. It depends on the Keywords I entered. According to Adsense's TOS, Adsense can not be alternative ads of other Ads, but other Ads can be alternative ads of Adsense. So I assume Adsense banned some links on http://www.digitalpoint.com/tools/adsense-sandbox/, such as http://www.digitalpoint.com/tools/adsense-sandbox/?url=real+estate On that search query, even you refresh it severa times, you don't get adsense displays.
Google does not ban them... but I'm sure there are some reasons that it doesn't always give results (usually if you do the same thing over and over).
I just wonder if you alter the ad display from your server end, or from adsense server end? https://www.google.com/adsense/policies Alternate Ads If you have elected to receive content-based ads, you can make sure that your advertising space is always being used effectively, either by targeted AdWords ads, or by your own choice of content by specifying an image or ad server of your choice. However, you may not specify Google ads as your alternate ads.
I know that Google has a list of 'sensitive' words that have to get manually aproved. I ran a test campaign for a client, and all the phrases with the word 'baby' in them were held back for approval.