Error handling expects legacy error codes
For example when joining a MUC, if the server does not exist, or some other error occurs, the reply error stanza triggers Gajim's auto-rejoin code to attempt rejoining the room. This causes an error loop between Gajim and the server, potentially flooding both the user and server with stanzas. If the reply error from the server contains code='404' however than Gajim will show a more friendly dialog informing the user of the error.
Some servers (including Prosody) do not attach legacy error codes (such as 404) which Gajim looks for to bounced stanzas. Legacy errors are now only documented in a deprecated XEP, and I would much rather see Gajim updated in line with the RFCs than add a deprecated protocol feature to Prosody.
The relevant XEP is here: http://xmpp.org/extensions/xep-0086.html The new RFC-defined errors are here: http://tools.ietf.org/html/draft-ietf-xmpp-3920bis-02#section-9.3.3
(PS. Johnny, this is now the only bug I have to file, thanks!)