Sudoku

I'm filling in numbers from 1 to 9 in each box. There is only that one special number that can go in that box in order for me to fill out all the rest of the numbers in all the other boxes, because each row and column of boxes can only have that number once.

I've been given a few numbers scattered around the puzzle to start off with. I'm taking my time, making sure to put in the right numbers in the right box. You don't fill in the box with a number unless you are sure of it. Slowly the puzzle starts to fill with numbers, and as there are less and less empty boxes to fill out, the puzzle seems to get easier, soon it will come to an end. Everything looks set and you feel like you're on track to finish it.

Then suddenly, as you are about to fill in the next box you stumble upon a number you cannont fill in because it conflicts. Fuck, it's so stupid because a few minutes ago you were so sure that the number was supposed to go into that box before you filled it in. Now the puzzle doesn't work out no more and you got to erase everything because all the other boxes that you filled in before were all dependent of that box that you filled wrong. Now I gotta start all over again.

How could you be careless in life?