> First, during mirroring the data transfer rate and 
> the number of links scanned are shown with 
> extra values in brackets, so:
>     Transfer rate      100 KiB/s (x KiB/s)

realtime tranfer rate (average transfer rate)

>     Links scanned     50/172    (+ y).

50 links downloaded and validated so far, 172 links 
detected, y links downloaded in background but not yet 
checked.

> Then I use HTTrack to mirror from the list. I have
> discovered that I get many more "Service not available
(503)"
> errors using this list than when I use a text file with
> exactly the same URLs, but without the "http://" prefix.
> Is there any explanation for this phenomenon?
Humm, no. These ones are treated exactly identically by the 
engine. I think you experienced some server oddities at the 
same moment, but I doubt this was related to the way you 
wrote the URL list.