Found this while doing related other testing - the setup (I think) is:

1) Start a replica set with 3 (regular) nodes
2) Introduce a 40s (asymmetrical) network delay between one secondary and the primary, causing the secondary to timeout when trying to reach the primary. The primary can still see the secondary.
3) Start a number of inserts into the primary
4) Eventually it seems the delayed secondary attempts to elect itself primary and succeeds because the other secondary votes for it. This does not trigger a stepdown of the original primary.

Two logs and scripts to reproduce below. All testing was done localhost, but using minor modifications of the test framework to assign each host a different local IP.