Could this be an alternative link? Unique Rectangles
The link is saying that if you end up with the 2 blocks like this (the four pink boxes can only have either 1 or 2 in them, "MadOverlord" named this Unique Rectangle pattern the Deadly Pattern apparently!):
- the setter has not created a sudoku with a unique solution (since there are two possible ways to populate the boxes)
- or you have gone wrong somewhere.
The writer from Sudoku Wiki goes on to say that if you know that there is a unique solution then you can use potential blocks like this to your advantage when solving the puzzle.
For example, in the below:
You will be able to see that you cannot place a 7 in any of the blue boxes as this will leave behind the deadly pattern, and so the 7 must be in the bottom left of that rectangle.
There are a few variations on this example on that page - You can draw conclusions about the numbers in more than just the four rectangle boxes themselves.
Personally, I am a bit wary of using this method as making any assumption in a sudoku makes me feel a little uneasy! It was an interesting read though.
To answer you question, I don't think that finding that you have "the deadly pattern" is the mistake, but it shows you that maybe somewhere earlier in the solving, you've made a mistake. Or else, there are multiple solutions. As we can see from this question, not many people like multiple solutions:
Why must a sudoku have a unique solution
I hope that helps? Or at least that the sudokuwiki link helps!