I was banned at one time by Yahoo, but was able to get back into their SERPs after pulling some strings.
Yahoo never told us exactly what we were doing wrong, but as far as we knew, we weren't doing anything illegal on our website.
However, I was a bit uncomfortable not knowing why we were blocked, so I did some more checking. i used an online/web-based cloak checker and it says our site "could be cloaking" since the page sizes differed between a browser agent and a simulated googlebot agent.
I compared the two web pages the online utility displays and the only difference I see is the banner ads - which obviously differ in size. We use a randomizing function which will display a different banner each visit - hence the different file size.
This is probably why some Yahoo auto-mechanism banned us, but i am not sure. My question is, "Are random banner ads considered cloaking and if so, how do I get it so my pages are exactly the same size?" It doesn't seem logical since many sites use random banner ads.
So maybe I shouldn't worry about the different filesizes?