Even more strangely, I have seen items labeled as 'panels' with one person scheduled (albeit not at a Worldcon - that I can recall). Hence my question.

For bonus unrelated fun, however, I have also attended panels where zero panellists turned up, but there was a room full of audience. Including several people more qualified to talk on the topic than the original panellists. No prizes for what happened next. (Again, not a Worldcon.)