Something I've never experienced with Gnus happened while reading the
newsgroup gmane.linux.suse.general: the threading suddenly fragmented
and one article appeared three times, once dangling before its thread
and twice within the thread.  When it happened I had most likely just
typed RET or `n' to read the next unread article, though I don't
remember if this was on the article that became dangling.  Here is a
screenshot of the fragmented threading: