Often when you are upgrading or migrating content to a new MOSS site, you will inevitably use the 'STSADM -o export' and 'STSADM -o import' commands. One of the 'traps' when using the export command is if the site you are exporting contains subsites, if you do not have access to that subsite (i.e. the subsite has not inherited permissions from its parent), then the subsite will not be exported (and hence will not exist after the import).
The other thing to be wary of is the use of the -includeusersecurity switch. If you do not use this switch in the export, and then again in the import, all content within the imported site will list you as the author. The downside in using this switch however is that all of the old security settings that existed in the exported site (e.g. WSS v2 Site Groups), will exist in the imported site, so you will need to do a cleanup afterwards.