When one of our secondaries is resynced, it appears that C# driver sends slaveOk queries to it while it's still recovering.
There are several errors in the database log like this one:
Mon Mar 19 21:21:03 [conn328] assertion 13436 not master or secondary, can't read ns:d2_feed3.counters.car
And, of course, the query fails on the client side.