I am trying to figure out exactly what causes 
the "file has moved from URLa to URLb" or "file has 
temporarily moved" error message.

It is happening on the first thread of a mirroring 
job, so it is not a multi-thread issue.  

I am assuming this is some sort of anti offline 
browsing trick, but I am curious as to what exactly it 
is detecting and if there is any way around it.  How 
does the server know I am using an offline browser?