I published the puzzle below in 2014:
Look at the 3x3 box or mini-grid number 6 i.e. the one with the clues 3, 4 and 8.
Where does the number 1 go in this box?
It cannot go in row 4 as there is already a 1 in row 4 column 4.
It cannot go in row 6 as there is already a 1 in row 6 column 1.
It must therefore go in row 5.
It cannot go in column 8 as there is already a 1 in column 8 row 2.
It must therefore go in row 5 column 7 as there is nowhere else for it to go.
This is called a hidden single in a box and the puzzle becomes:
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