Not the best attempt

I usually don't make mistakes when solving a sudoku in the newspaper. Especially not in one of both, because they are quite simple. Even although it says "Fortgeschrittene", i.e. advanced. But this is a silly mistake. Well, that suits the challenge of today, Silly Saturday :(.

For those who don't see it:
In sudoku every row and every column contains the digits 1 to 9, and only one of each. The same is true for every of the nine squares.
The ninth column had an eight in the top square, the eighth and seventh not yet in either of the two other squares. The middle square could only get an eight in the eighth column (and hence the bottom square only in the seventh column).
The fourth row had an eight in the middle-left square. Hence the eight in the middle-right square could only be in row five or six.
But why not in row five, where I put it? Well, if row four has an eight in the middle-left square in the fourth row and an eight in the middle-right square in the fifth row, then the eight in the middle-middle square can only be in the sixth row (one per row).
But if you look in the middle-top square you see an eight in column 6, and in the middle-bottom square in the column 5. Hence in the middle-middle square an eight can only be in column 4.
However, column 4 and row 6, that field is already occupied!