3 members in a replica set.
Go to secondary1 , rs.syncFrom('secondary2')
Go to secondary 2, rs.syncFrom('secondary1')
Result: replica set now configured such that secondaries sync from each other, none of them from the primary.

This situation is detectable in the status, and at least should produce a warning when attempted "sync target specified appears circular".

Since RS and elections may break this circularity, it may be allowed, but to an unsuspecting user this may cause confusion since the secondaries will "fail" to update.