diff --git a/source/faq.txt b/source/faq.txt
index 8ab2775c0..ad7fae829 100644
--- a/source/faq.txt
+++ b/source/faq.txt
@@ -30,6 +30,11 @@ If you write to a synced namespace before issuing a :ref:`commit
 undefined. To ensure that you don't write to any synced namespaces, enable 
 :ref:`write blocking <c2c-write-blocking>`.
 
+.. note::
+   
+   Index builds on the destination cluster are treated as writes 
+   while ``mongosync`` is syncing.
+
 To check the value of ``canWrite``, call the :ref:`progress <c2c-api-progress>` 
 API endpoint.