It may take a moment
This is one of those annoying things that you don’t really notice at first, but it can drive you crazy when you do catch it. You can’t take your eyes off it, and you want to jump in and fix it.
The person who owns this house shared the picture with a caption that they didn’t notice anything wrong with the stairs for five years. In case you are having trouble locating the odd section, just look at the middle bar. It’s upside down.