[sp332]

Well if you switch the squares on row 3, you have to switch the square on row 1 as well. Otherwise the columns will not have even numbers of red and blue squares.

[anon4]

Oh. It's really confusing to show not only the current mistake but also the mistakes that would be introduced by fixing it.

[sp332]

True. There is only a single solution to each puzzle, so it just highlights all the blocks which don't match the known solution.