The partial-collision sentence, copied verbatim

The scalar-sourced coordinate part of $\mathcal{O}$ contains a branch with schematic form

Two scalar contractions can join the stress slots at $z$ to the two scalar slots of the interaction at $y$. The collision $y=z$ may occur while $x\ne z$. A counterterm supported only where $x=y=z$ does not by its support alone normalize this internal collision.