I have my ads shown on a sidebar on every page on my forum including the login page, permission error page, and etc. Almost every forums does the same thing but in a different layout. Is there a chance for me to get in trouble?
I personally only show ads on pages with content where I can use section targeting to get relative ads but I wouldn't worry about it in your case. As far as error pages go, what I do is show the most popular threads. That seems to be great for SEO and indexing purposes.
As long as you don't show more then the allowed number of ad units per page you should be OK or have I missed something?