It doesn't seem you can lose data even when you might otherwise expect to ;)

I setup a small replica set using

mongod --fork --logpath a.log --smallfiles --oplogSize 50 --port 27001 --dbpath data/z1 --replSet z
mongod --fork --logpath b.log --smallfiles --oplogSize 50 --port 27002 --dbpath data/z2 --replSet z
mongod --fork --logpath c.log --smallfiles --oplogSize 50 --port 27003 --dbpath data/z3 --replSet z

And initalized it:

> rs.initiate(
    { _id:'z',
      members:[
        { _id:1, host:'localhost:27001' },
        { _id:2, host:'localhost:27002' },
        { _id:3, host:'localhost:27003' }
      ]
    }
);

Then I killed all three processes:

kill -9 25542 25496 25483

Next I brought one of them back up

mongod --fork --logpath c.log --smallfiles --oplogSize 50 --port 27003 --dbpath data/z3

and inserted a doc

> db.foo.insert({a:1})

Then I killed that process

kill -9 25885

and brought the replica set back online using

mongod --fork --logpath a.log --smallfiles --oplogSize 50 --port 27001 --dbpath data/z1 --replSet z
mongod --fork --logpath b.log --smallfiles --oplogSize 50 --port 27002 --dbpath data/z2 --replSet z
mongod --fork --logpath c.log --smallfiles --oplogSize 50 --port 27003 --dbpath data/z3 --replSet z

and my data is still there:

> mongo --port 27003
MongoDB shell version: 2.4.6
connecting to: 127.0.0.1:27003/test
z:SECONDARY> db.setSlaveOk()
z:SECONDARY> db.foo.find()
{ "_id" : ObjectId("52eb09f9df67edd1c6d33e71"), "a" : 1 }
z:SECONDARY>

In addition, if I insert data into the primary it seems to be perfectly happy to live right there alongside my existing data on the secondary:

> mongo --port 27001
MongoDB shell version: 2.4.6
connecting to: 127.0.0.1:27001/test
z:PRIMARY> db.foo.insert({b:2})
z:PRIMARY> db.foo.find()
{ "_id" : ObjectId("52eb0cefdf3e83d897ae68ec"), "b" : 2 }
z:PRIMARY>

> mongo --port 27003
MongoDB shell version: 2.4.6
connecting to: 127.0.0.1:27003/test
z:SECONDARY> db.setSlaveOk()
z:SECONDARY> db.foo.find()
{ "_id" : ObjectId("52eb09f9df67edd1c6d33e71"), "a" : 1 }
{ "_id" : ObjectId("52eb0cefdf3e83d897ae68ec"), "b" : 2 }