In my paper, [A.Goel, Stability of networks and protocols in the adversarial queueing model for packet routing, Networks, 37(4):219--224, 2001], I characterized (among other things) the set of excluded minors for digraphs which are universally stable, and also digraphs which are stable for FIFO. I neglected to mention that this was only true for digraphs without parallel edges.
The following publication rectifies this, and presents nice results about instability of graphs when packets are not allowed to follow simple paths, and when graphs are allowed to have parallel edges.
Departament de Llenguatges i Sistemes Informàtics, Universitat Politècnica de Catalunya, Technical Report.
Report Number: LSI-02-4-R
Title: A characterization of universal stability for directed graphs in the adversarial queueing model
Author(s): Alvarez, C., Blesa, M., Serna, M.