Thank you for the report. This isn't an issue of an advertisement obscuring the page per-se, the problem is that one of the posts in that thread contains broken BBCode which has broken the layout of the page. I have repaired the post and the advertisement is now correctly placed, and the forum footer is visible. Thanks!
Recurring bug, seems to be page specific.
Using Google Chrome 35.0.1916.153 m.
Link to page
http://www.minecraftforum.net/forums/off-topic/forum-games/435676-destroy-the-godmodder-2-operator-scratched-rip-in?page=533
Not quite sure if you can reproduce it that way. (However it always occurs for me)
How I got around it:
I went to another page where the ad didn't obscure the page.