Upgrading to MOSS 2007 or WSS 3.0 can cause a lot of problems. Before doing so, it is a good thing to determine where issues can occur and how to fix them. A new tool from Microsoft can help you with this, the Upgrade Pre-scan Tool.
"Before you can upgrade to Office SharePoint Server 2007 or Windows SharePoint Services 3.0, you must run the Prescan.exe utility against your existing Web sites. This pre-upgrade scan tool scans your sites and reports issues, allowing you to fix any errors before you perform an upgrade. If you have not successfully run this tool and you attempt to upgrade your environment, when you attempt to run the SharePoint Products and Technologies Configuration wizard, the wizard will exit and prompt you to run the tool. We highly recommend that the server administrator run the pre-upgrade scan tool before the upgrade and resolve any problems that can be resolved before scheduling the upgrade."
Download the documentation and the application here.