Add "what do I do if something goes wrong during buildworld" FAQ

to cutting-edge chapter.

PR:		31803
Submitted by:	mwlucas@blackhelicopters.org

en_US.ISO8859-1/books/handbook/cutting-edge/chapter.sgml

@@ -1580,6 +1580,35 @@ Building everything..
 	    </itemizedlist>
 	  </answer>
 	</qandaentry>
+
+        <qandaentry>
+          <question>
+            <para>What do I do if something goes wrong?</para>
+          </question>
+
+          <answer>
+            <para>Make absolutely sure your environment has no
+              extraneous cruft from earlier builds.  This is simple
+              enough.</para>
+
+            <screen>&prompt.root; <userinput>chflags -R noschg /usr/obj/usr</userinput>
+&prompt.root; <userinput>rm -rf /usr/obj/usr</userinput>
+&prompt.root; <userinput>cd /usr/src</userinput>
+&prompt.root; <userinput>make cleandir</userinput>
+&prompt.root; <userinput>make cleandir</userinput></screen>
+
+            <para>Yes, <command>make cleandir</command> really should
+              be run twice.</para>
+
+            <para>Then restart the whole process, starting
+              with <command>make buildworld</command>.</para>
+
+            <para>If you still have problems, send the error and the
+              output of <command>uname -a</command>to &a.questions;.
+              Be prepared to answer other questions about your
+              setup!</para>
+          </answer>
+        </qandaentry>
       </qandaset>
     </sect2>
   </sect1>