For a long time ask.com had one of the only Google ad feeds allowing them to programatically request ads from Google to show on their search pages and for some reason instead of implementing it themselves they used a company I worked for to do it so for some time a lot of the ads on ask.com were actually google or yahoo ads running through a random ad server I wrote. I remember having to move our systems to make sure we were in a data centre as close as possible to them and Google/Yahoo since we had (I think?)50ms to receive a request from them, contact google and yahoo for ad inventory, merge them and return it to ask to show on the page.
(This was all like 15 years ago now)
I'd love to see a write-up of this if you ever get the chance.
There really isn't too much more to it but happy to try and answer any specific questions. I wasn't involved in the business dealings at all so I have no clue why it happened. System was originally written in PHP and I later rewrote it in Erlang as we got more sources so I could contact all the networks for ads at the same time. It was a very lightweight system the click handler was the heavier one.