According to the PDF specification:
A clipping path operator (W or W*) may appear after the last path construction operator and before the path-painting operator that terminates a path object.
Invalid:
W % clipping path operator 0 0 m % start of the path construction 596 0 l 596 842 l 0 842 l h % end of the path construction n % path-painting operator
0 0 m 596 0 l 596 842 l 0 842 l h W n
Currently (in the invalid cases), the path construction is skipped on import.