diff --git a/doc/source/install/mod_wsgi.rst b/doc/source/install/mod_wsgi.rst
index e75979317..950978ba0 100644
--- a/doc/source/install/mod_wsgi.rst
+++ b/doc/source/install/mod_wsgi.rst
@@ -54,3 +54,16 @@ work with a copy of ceilometer installed via devstack.
 
       $ a2ensite ceilometer
       $ service apache2 reload
+
+Limitation
+==========
+
+As Ceilometer is using Pecan and Pecan's DebugMiddleware doesn't support
+multiple processes, there is no way to set debug mode in the multiprocessing
+case. So user will run into HTTP 500 error if the mod_wsgi's multiprocessing
+is enabled and the Ceilometer debug mode is enabled at the same time. There
+is no good way to make both of them work, since Pecan is sharing the debug
+mode with Ceilometer, see[1]. If you really need to enable both, a possible
+workaround is hacking that line and hardcode the debug configration to False.
+
+[1] https://github.com/openstack/ceilometer/blob/master/ceilometer/api/app.py