With a stack of rows of water dispensers, each powered by redstone dust running across the top of each row of dispensers, the bottom row of dispensers will get stuck. In this case, they never dispense their water. Usually they dispense water once and then never suck it back in, but in this test world I accidentally powered the dispensers once before putting the water in, so they never dispensed their water in the first place. In the world in which I discovered this bug, I put the water buckets in before powering the dispensers the first time, so the bottom dispensers there were stuck with their water out.
I think that the dispensers are staying stuck in a powered state after power is removed. Since dispensers must go from "off" to "on" to do anything, being stuck on is the same as being stuck off.
The obvious workaround is to put an extra line of redstone dust below the bottom row of dispensers.