Putting an observer on a piston facing horizontal and then placing water on the observer will freak out the water updates. See images below (untitled1 is when it updated, untitled2 is when it did not update)
Note: It is random when the water will update for some reason, but earlier it would work every time I put a wool on it.
Environment
macOS Sierra, latest Java, iMac Late 2013, 3.5 GHz Intel Core i7, 8gb memory, NVDIA GeForce GTX 780M 4096 MB
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