> This is a question regarding updating an existing 
> mirror. If during an update a previously mirrored 
file 
> is referenced (as it was in the original mirror) but 
> for some reason (maybe server trouble or file is 
> temporarily unavailable) the server returns a 404 
> error for the file, HTTrack overwrites the original 
> file with a 404 message file. This is done even if 
> the -X0 option is given.
> Is there any way to prevent this?
No - the engine always tries to match exactly the 
remote site, and will erase local files if remote ones 
were erased - or missing. You can activate the 'do not 
erase local files' option, but this won't work if the 
file is still linked, and missing remotely (or in 
error). I may add an option in the future however, 
which will try to avoid that.. but this won't be quite 
easy to implement (without breaking th cache)