I think we've seen this before, so this is probably just the latest example.
This Ready Room thread about our favorite TV/Movie quotes is now up to 11 pages:
http://cs.finescale.com/fsm/general_discussion/f/50/t/175273.aspx?page=11
When I click the link in the index page, http://cs.finescale.com/f/, to open the thread, the page displays this error:
I can refresh the page with the browser refresh control or F5, and then it loads successfully.
When I added a reply and clicked Save, the page threw off the error again. Again, reloading the page gets around the error.
Not a business critical issue, because we have a workaround, and we don't have too many threads that are that long or longer. But we do have some, and the forum's software should be able to handle this case.