What mistake?

Jennifer's post entitled "I love jigsaw puzzles" jogged my memory that I have a similar jigsaw puzzle (undone) lying somewhere in the house. After searching for a while, I finally found the puzzle that I bought 3 years ago. Without further ado, I started putting the pieces together.

As with the usual practice, I put together the border first. To make it a bit more difficult, I only looked at the picture on the cover of the box once and then proceed to put the pieces together without referring to the picture again.

When I reached this stage, it was getting harder and harder to find the right piece as each small piece contains a variety of colours so without referring to the complete picture, it was difficult to know which piece goes where but yet I persisted to do it without referring to the picture again.

Finally I did it! The key to putting the puzzle together without referring to the picture again is to group the pieces that have roughly the same colours together and then assemble the pieces by their groups.

Maybe I should complete a black and white jigsaw puzzle this way which should be very difficult. Or maybe I should ask my spouse to get me a jigsaw puzzle and let me try putting it together without me knowing what  picture is at all. The latter should be more difficult. But more fun!

For this puzzle, I am supposed to find the mistake but after scrutinising it for hours, I still couldn't figure out what the mistake is. What mistake?!