Re: a serious bug in "mv"

   I simply do not believe this behavior is intentional, nor "as
   planned".

It is the correct behaviour. "mv Dir/*.*" gets expanded (by the shell)
to "mv Dir/A.txt Dir/B.txt", and this is what you got.  If you'd like
to be warned of what mv does, you can supply the -i option to it,
which will ask for confirmation before each file it moves.

Sorry for your loss though...