I started using Bing recently and I noticed that I was getting error pages intermittently, and then pretty frequently.

phpbb and topic view are returning error pages for me (on latest ff, cookies enabled).

Another way I'm reliably getting an error page is adding parameter &first=201, e.g. http://www.bing.com/search?q=stuff&first=201

This is the paging parameter, indicating you're at the 20th screen of results.