  A mixed s-stack q-queue layout of a graph consists of a linear order of its
vertices and of a partition of its edges into s stacks and q queues, such that
no two edges in the same stack cross and no two edges in the same queue nest.
In 1992, Heath and Rosenberg conjectured that every planar graph admits a mixed
1-stack 1-queue layout. Recently, Pupyrev disproved this conjectured by
demonstrating a planar partial 3-tree that does not admit a 1-stack 1-queue
layout. In this note, we strengthen Pupyrev's result by showing that the
conjecture does not hold even for 2-trees, also known as series-parallel
graphs.
