Occasionally getting the error :"the web page at might be temporarily down or it may have moved permanently to a new web address"

We have a customer that frequently gets the error, "the web page at might be temporarily down or it may have moved permanently to a new web address" when accessing our web site. We had seen it once in a long while but aren't sure what's causing it. We're using an IIS web server connecting to SQL Server on a Windows 2008 RS server. Is this something in the IIS configuration?.. or does anyone have a clue where we can look, what causes it, etc.?