There has been an error while loading the form. A required resource could not be downloaded. To To try to resume the download, refresh the page.
You will also receive the following in your SharePoint ULS logs straight after the user sees this error regarding the "Canary" timing out:
10/07/2010 10:53:54.89 w3wp.exe (0x0D70) 0x1804 Forms Server Forms Services Runtime 5ajc Medium The Canary has timed out for form 6bbd1ceb-7956-49de-aaa5-015d7d94d2b2:ver:18.104.22.1688
10/07/2010 10:53:54.90 w3wp.exe (0x0D70) 0x1804 Forms Server Forms Services Runtime 7tel Assert WARNING: Invalid Canary for view file. StackTrace: at Microsoft.Office.InfoPath.Server.Controls.ResourcePage.GetSolutionAndVerifyCanary(HttpContext context, String solutionId, SPSite contextSite, ResourceErrorType& error) at Microsoft.Office.InfoPath.Server.Controls.ResourcePage.HandleViewFile(HttpContext context) at Microsoft.Office.InfoPath.Server.Controls.ResourcePage.<>c__DisplayClass2.
This is a result of the session timing out (the default is 20 minutes). Simply increase the session time out based on your user's standard behaviours (e.g. going out to lunch while filling out a form or taking a very long time to fill out a form). We increased ours to 1 hour session timeout as described here