A. This, like many other things  commonly referred to as "paranormal",
is to  be considered unknown.   There is much  evidence against it, it
would contradict the laws  of nature as  recognized by most scientists
today.  (Any information getting from   future to past would have   to
break the  speed of light, which is   impossible. More on this  can be
found in the  sci.physics FAQ postings.)   However, many people insist
on having experienced "deja-vu" like situations where they came into a
setting they  already had dreamed of.  Could  they prove it?  Probably
not   (cf.  section   5.3)  but  this  fact alone   doesn't  prove the
experiences invalid.    (Proving  a subjective  experience  *wrong* is
impossible.)

Now,  how come  the  many deja-vus? A   common explanation  is a small
misfunction of the brain. Some piece  of information, like the look of
a particular place where you haven't been before, gets from short-term
into  long-term memory  via some  sort of "shortcut"  rather  than the
usual remembrance mechanism. (How this exactly  works is currently not
known in detail.)  Then,  when matching short-term against   long-term
memory, you  think that you  have  the piece  in long-term memory from
somewhere in  the past  while   it has entered long-term   memory just
recently.

This could explain  some of  the instances, but the possibility of the
mind  "travelling  in time"  (or place, cf. section  4.1)  can  not be
dismissed - many people claim they have done it and can do it again.