Yep, I guess my index was broken. I did:

sup-dump > index.dump
mv .sup/xapian .sup/xapian-bak
sup-sync -a
sup-sync -o -r --restore index.dump

All my mail is where I left it and I was able to spam those threads. No idea
what caused the corruption.

Looks like your index is somehow broken.  Have you tried recreating it?  Further
instructions found in http://masanjin.net/sup-bugs/msg159

I have three spam threads in my inbox. If I try to interact with them in any way
- view them, spam them, delete them, etc. - sup crashes.

I'd appreciate any suggestions for how to get rid of them.

--- RuntimeError from thread: index sync
DocNotFoundError: No termlist for document 2362106147
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:600:in `old_add_term'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:600:in `add_term'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:510:in
`index_message_labels'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:510:in `each'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:510:in
`index_message_labels'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:446:in `sync_message'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/xapian_index.rb:98:in
`update_message_state'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/index.rb:205:in `run_sync_worker'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/index.rb:192:in `start_sync_worker'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup.rb:77:in `reporting_thread'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup.rb:75:in `initialize'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup.rb:75:in `new'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup.rb:75:in `reporting_thread'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/index.rb:192:in `start_sync_worker'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/index.rb:236:in `send'
/home/harkins/.gems/gems/sup-0.10.2/lib/sup/index.rb:236:in `method_missing'
/home/harkins/.gems/gems/sup-0.10.2/bin/sup:170
/home/harkins/.gems/bin/sup:19:in `load'
/home/harkins/.gems/bin/sup:19