+manpagesection(DIAGNOSTICS)
+
+rsync occasinally produces error messages that may seem a little
+cryptic. The one that seems to cause the most confusion is "protocol
+version mismatch - is your shell clean?".
+
+This message is usually caused by your startup scripts or remote shell
+facility producing unwanted garbage on the stream that rsync is using
+for its transport. The way ot diagnose this problem is to run your
+remote shell like this:
+
+verb(
+ rsh remotehost /bin/true > out.dat
+)
+
+then look at out.dat. If everything is working correctly then out.dat
+should be a zero length file. You you are getting the above error from
+rsync then you will probably find that out.dat contains some text or
+data. Look at the contents and try to work out what is producing
+it. The most common cause is incorrectly configued shell startup
+scripts (such as .cshrc or .profile) that contain output statements
+for non-interactive logins.
+