I received a message on a mailing list which the sender's software had
encoded in a strange way.  The message looked like this:

[1. application/x-pkcs7-mime; smime.p7m]...

I decided to select 'view as type' from the mouse-3 menu on the above
button, and said 'text/plain'.  Then I could see some trash followed
by the message. (I can't post it because it says  "Multibyte character
in QP encoding region" - because of the 'trash', no doubt).

However, doing this hosed thing a bit in that pressing more or less
any key (q, for example) resulted in:

Wrong type argument: listp, "smime p7m"

All I could do was kill the Article _and_ Group buffers

Here is the backtrace I got when trying to quit the Article buffer: