Water in a U shaped area doesn't properly drain from the area applying the pressure.
code:
z5 #~#
z4 #~#~~
z3 #~#~#
z2 #~~~#
z1 #####
This structure (side view) would never run out of water and eventually flood
everything up to z4.
Demonstration video: http://mkv25.net/dfma/movie-207-infinitewaternopumps