
This is an invalid puzzle.
It has two possible
solutions. There is no way to remove a 4 or
7 so both solutions are possible.
By definition, a deadly rectangle exists when four
identical bivalue cells are confined to:
 two columns
 two rows
 two boxes
An easier way to think of this is that the four cells form
a rectangle (but don't forgot about the two boxes!).
The four cells involved cannot reside in four different boxes.
Since deadly rectangles can only occur in invalid puzzles, you should never
encounter a deadly rectangle when working a valid sudoku puzzle. So the rule is this:
"Any move that completes a deadly rectangle is wrong". If you encounter a deadly rectangle and
continue working the puzzle, you will encounter a dead end (a cell with no possible value). A deadly
rectangle cannot exist in a valid sudoku puzzle.
There are several solving techniques that work
by avoiding deadly rectangles

