I notice this strange little bug every so often. It involves three Emacs modes: Wanderlust (a mail client), ElScreen (a GNU Screen-a-like for managing Emacs window configurations), and ERC (an IRC client).

ElScreen comes with a handy add-on for Wanderlust which includes a feature to allow composing drafts in their own new screen. When you send your message (C-c C-c), the new screen which it opened is killed and it takes you back to the last screen you were looking at.

I've noticed that, if the last screen I was using before sending my email was an ERC buffer containing an IRC channel, when it returns to that screen after sending the message, it makes me /leave the channel. It's not even like ERC's erc-part-from-channel function (bound to C-c C-p); that prompts you for a reason before leaving.

Very weird.