If you start 3 servers and then set slaveOk on one of the slaves, then kill the primary, there are a couple seconds where querying the slave doesn't work:

> b1.getMongo().setSlaveOk()
> b1.bar.find() // querying the slave works
> b1.bar.find()
> b1.bar.find()
> b1.bar.find() // kill the master
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find()
error:

{ "$err" : "not master", "code" : 10107 }

> b1.bar.find() // querying the slave works again
> b1.bar.find()
> b1.bar.find()
> b1.bar.find()

I'm having a hard time writing an automatic test that reproduces this, but I'll commit it when I get it working.