Steps to reproduce:

Create a replica set with 6 members. Bring down 3 members. Before the primary demotes itself to secondary, reconfig to a configuration that includes the 3 up members and 1 down member. The reconfig will succeed. If you do the same thing but wait for the primary to demote itself to secondary, the reconfig will fail (unless you use force:true).

This may not actually be a problem as it seems that the reconfig will only work if it's changing to a configuration that will have a majority.