We had the following situation:

Server A: Primary
Server B: Down
Server C: Secondary

We then added a fourth server, D, to the replica set with a view to removing server B afterwards.

This caused server A to step down as it no longer had a majority - which is technically accurate.

However, I think MongoDB should be smarter than this. We're adding a brand new server - there is no way that the new server should become primary. Alternatively, there should be a warning when adding the new server that the primary will step down and the set will not elect a new primary.