If you're attempting an STSADM -o export (or, I'd assume, a content deployment) of a site and receive the following error:
FatalError: Failed to compare two elements in the array.
Check out the following KB article: http://support.microsoft.com/kb/948726/en-us .
The article indicates that the likely cause is one or more invalid feature references in the SPSite and/or SPWeb object. Fortunately, Steven Van de Craen authored a wonderful GUI tool for removing "faulty features" (which is of course unsupported and should be used with caution, but nonetheless very useful). See his article on the topic for more details.
Cases where you might encounter these errors: