Possibly relevant: some implementations parse the directory in the "/tmp/x/../y/" namestring as '(:absolute "tmp" "x" :back "y"), yielding #p"/tmp/y/", whereas SBCL parses it as '(:absolute "tmp" "x" :up "y").
Possibly relevant: some implementations parse the directory in the "/tmp/x/../y/" namestring as '(:absolute "tmp" "x" :back "y"), yielding #p"/tmp/y/", whereas SBCL parses it as '(:absolute "tmp" "x" :up "y").