I like to build wells in my hospitals so the doctors don't have to go too far to get water. At first, what I did was build a cistern underneath the hospital and tap it into a river (or other water source), using a floodgate linked to a lever to cut off the cistern from the water source before it flooded (I don't use the diagonal baffles in this case, because they slow down the flow too much and I want the cistern to fill quickly). The problem with this set up is, when they use up all the water in the cistern, you have to pull the lever to let it back in, then pull the lever to close it back off. Too much supervision required.
So, what I did this time was put a pressure plate in the cistern linked to a bridge that, when raised, would block off the cistern. I set the pressure plate to be activated only by 7/7 water. When I first tapped into the river, the cistern filled with some slight (but inconsequential) flooding in the hospital before the bridge raised.
So far, so good. The problem came when the water level dropped enough for the bridge to drop. It quickly refilled, and then...
...well, you know how if you pull and re-pull a lever to quickly, how one of the signals gets missed by the engine? Yeah, well, that happens with pressure plates too. The bridge never raised again.
Three drowned dwarves later, I remembered that I had an emergency floodgate between the river and the cistern, in case of such a mechanical failure.
Next time, I'm gonna set the pressure plate to lower the bridge at 4/7, and raise it at 6/7. Learn from my mistakes, folks!