AFAICS, the given message-id appears in the References header of 2 messages,
nowhere else.

Inspecting the offending Thread object:

Searching for threads...
#<Redwood::Thread:0x8e0a408 @containers=[#<Redwood::Container:0x8e0a574
@id="p06240807c6b098681dd5@192.168.11.2", @thread=#<Redwood::Thread:0x8e0a408
...>, @parent=nil, @message=nil, @children=[]>]>
[2010-01-24 23:35:06 +0100] ERROR: oh crap, an exception

This happened after sup-convert-ferret-index. Started fine before.

Appearently, t.first is nil in the given line in thread-index-mode.rb.
Interestingly, t.empty? is not true.

--- RuntimeError from thread: load threads for thread-index-mode
wrong id called on nil
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup.rb:17:in `id'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:226:in
`block (2 levels) in update'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:226:in
`each'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:226:in
`sort_by'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:226:in
`block in update'
<internal:prelude>:8:in `synchronize'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:224:in
`update'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:642:in
`load_n_threads'
(eval):12:in `load_n_threads'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup/modes/thread-index-mode.rb:614:in
`block in load_n_threads_background'
/usr/local/lib/ruby/gems/1.9.1/gems/sup-0.10.1/lib/sup.rb:77:in `block in
reporting_thread'