Reading from secondary nodes in a replica set may block the application of replication write operations, because longer read operations may not yield appropriately.
High volume read operations on secondary nodes may cause the nodes to experience increased replication lag, which may make read operations return old data.
In extreme cases the affected node may become "stale". Stale nodes need to be resynchronized. If enough nodes in a replica set become stale availability may be impacted.
The preferred workaround is to suspend all read operations on secondary nodes.
Alternatively, the oplog size can be increased on secondary nodes. This is only a suitable workaround if the nodes undergo periods of no reads so replication can catch up.
MongoDB 3.0.0 through 3.0.3.
The fix is included in the 3.0.4 production release.