Cannot compare two elements within an array

I had this error message appear whilst moving between our development and staging environments. I had created a backup of the site I wanted to move and copied it over to the staging environment. Created the sites, there were four host headed sites to be created on staging to replicate our production environment. On three of the sites the stsadm -o restore command worked fine and on the fourth it would not. So after much scratching around comparing features, using utilities such as WssAnalyzeFeatures and FaultyFeatures, there was nothing obvious or indeed showing up as an error.

So to resolve the issue I used the stsadm -o export, to get a cab file and used the stsadm -o import -versions 2 and everything was fine.