rs.add({_id:<id>,host:"<host>",priority:<prio>});

creates fatal replication error. master shuts down replication. this happened multiple times on different master nodes.

[conn40] Sun Jul 25 00:27:36 replSet replSetReconfig config object parses ok, 7 members specified
[initandlisten] Sun Jul 25 00:27:36 connection accepted from xxxxxxxxx #68
[conn40] Sun Jul 25 00:27:36 replSet replSetReconfig all members seem up
[conn40] Sun Jul 25 00:27:36 replSet info saving a newer config version to local.system.replset
[conn40] Sun Jul 25 00:27:36 Assertion failure !sp.state.primary() db/repl/rs.h 184
[conn40] Sun Jul 25 00:27:36 replSet error unexpected exception in haveNewConfig() : 0 assertion db/repl/rs.h:184
[conn40] Sun Jul 25 00:27:36 replSet fatal error
[conn40] Sun Jul 25 00:27:36 replSet error fatal error, stopping replication