It's possibly no fault of your own or your forum. Typically this is an IE specific message that shows up when a script takes a certain amount of time to execute.
http://support.microsoft.com/kb/175500
Another suggested fix:
http://help.npr.org/npr/consumer/kbdetail.asp?kbid=501
You can have your users test these same pages in Chrome or Firefox (presuming that's not what they're already using) to see if the same problem occurs.