Offline messages sent by XMPP clients usually send valid XML characters. However, XMPP libs may allow clients to send invalid XML characters. Something like this:

Chat chat = getConnection(0).createChat(getBareJID(1));
chat.sendMessage("Test \f 1");

will produce the following exception:

[org.jivesoftware.wildfire.OfflineMessageStore.getMessages(OfflineMessageStore.java:190)] Error retrieving offline messages of username: agent
org.dom4j.DocumentException: Error on line 1 of document : Character reference "&#19" is an invalid XML character. Nested exception: Character reference "&#19" is an invalid XML character.
at org.dom4j.io.SAXReader.read(SAXReader.java:482)
at org.dom4j.io.SAXReader.read(SAXReader.java:365)
at org.jivesoftware.wildfire.OfflineMessageStore.getMessages(OfflineMessageStore.java:168)
at org.jivesoftware.wildfire.handler.PresenceUpdateHandler.initSession(PresenceUpdateHandler.java:212)
at org.jivesoftware.wildfire.handler.PresenceUpdateHandler.process(PresenceUpdateHandler.java:100)
at org.jivesoftware.wildfire.handler.PresenceUpdateHandler.process(PresenceUpdateHandler.java:153)
at org.jivesoftware.wildfire.PresenceRouter.handle(PresenceRouter.java:92)
at org.jivesoftware.wildfire.PresenceRouter.route(PresenceRouter.java:61)
at org.jivesoftware.wildfire.spi.PacketRouterImpl.route(PacketRouterImpl.java:75)
at org.jivesoftware.wildfire.net.SocketReader.processPresence(SocketReader.java:296)
at org.jivesoftware.wildfire.net.ClientSocketReader.processPresence(ClientSocketReader.java:57)