Ernie, the nuclear plant supervisor was about to go on his first vacation in almost 20 years. 
He left the foreman in charge of the control room ...
   
Ernie lectured again to the crew to be sure to watch all the gauges. He reiterated how to cool the core by adding water if a nuclear meltdown should threaten.  And he reminded everyone about the dangers of over-filling the core with water, to avoid explosive pressure buildup from the steam, etc.
   
Having explained all that, as he was leaving, he turned and reminded the crew,  "Just remember, you can't add too much water."   

~ Source Nowscape.com...   From a classic Saturday Night Live episode hosted by Ed Asner.