I cannot remember the number of times I've metaphorically said "Duh" and smacked myself on the forehead! It's the first cousin of the phenomenon known as "Eagle-eye hindsight." See also Murphy's third law (below).Murphy's First Law
If in any given situation anything can go wrong, it will go wrong.
Murphy's Second Law
If in any given situation any one thing goes wrong, everything else goes wrong, too.
Murphy's Third Law
No matter how strange the outcome of any given situation might be, you can always find some clever b***** who knew it would.