The following test case failed during a pytest-randomly workflow:

I was able to reproduce locally on main by running interactively. When running without a seed, I occasionally get 1 instead of 0. This is also reproducible when seeded:

>>> G = nx.path_graph(4)
>>> nx.connected_double_edge_swap(G, seed=140)
1

I tested with both 2.8.3 and 2.7 and was able to reproduce with those versions as well, so I don't think this is something recently introduced (though I haven't looked at blame or bisected).