What is incongruous about the branch Don Quixote selects to replace the lace shattered at his encounter with the windmill?

The branch Don Quixote selects to replace the shattered lace is a thorny acacia branch, which is incongruous because it is spiky and rigid, unlike a lace which is delicate and soft. This reflects Don Quixote's disconnect from reality and his tendency to see ordinary objects in a distorted way.