It shouldn't be possible, but it is lots of times.
"Full Path Disclosure", it's encountered as a bug.
An example you may seen, is the php errors, if error_reporting is on and there's something wrong..
As it's a bug, most scanners should report it (if they find it :d).
The abs path usually, can reveal so many things or even used in some kind of combination with another vuln like LFI (local file inclusion).
