I was spamming my twitter account accidentally. Actually it was my Jabber/Twitter gateway, which got an XMPP error message from a broken jabber server and tweeted the error, then sent me a confirmation, got another error message and tweeted it again. Infinite loop!
The main problem is, that the error message looks very much like a good message. That's a Jabber "feature". A good message looks like:
<message from="JID" to="JID">If that produces an error, then comes back:
<message from="JID">to="JID"type="error">...which looks very much like the original message, if you do not check for the type="error" bit and just forward the body to the twitter API.
<error code="500" type="wait">
<internal-server-error xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
TwiX has now been updated to version 0.4.8. Get it here.