Secondary indexes (i.e. all indexes other than _id) may be
corrupted on an initial sync if write operations are performed on
the sync source during the initial sync. This affects version 2.4.0
and is fixed in 2.4.1. See SERVER-9087 for more
Documents may be missing on a replication secondary after initial
sync if a high number of updates occur during the sync that move the
document (i.e., the documents are growing).
When stepping down a replica set primary, the primary’s connection
to clients is not automatically closed in 2.0.1. This means that if
the primary steps down while an application is issuing
fire-and-forget write, the application won’t necessarily fail over
until it issues a query. Applications that only issue safe writes
are not affected by this. The issue is fixed in 2.0.2. See
<https://jira.mongodb.org/browse/SERVER-4405> for details.