> I don't know if this is an intentional side-effect
> of 'assume', but it does make sense
> that 'shouldnotappear.html' is created as originally is
> was a .php and HTTrack has been told to make all .php
> into .html

Yes, this is a side effect of 'assume': without the assume 
option, the engine will 'ask' the server for file type 
BEFORE requesting the link and BEFORE writing it on the 
page ; which allow to rename the link on-the-fly, and 
therefore avoiding the intermediate page.