frozencemetery <rharwood@club.cc.cmu.edu> writes:
>
> To reproduce:
>
> 1. Open new buffer
> 2. Create new message with non-matching "-marks (this message will do)
> 3. M-x (show-ws-toggle-show-trailing-whitespace)
> 4. refresh buffer (=)

I think I managed to duplicate this without notmuch.

0. stash file name of that message (c F), quit emacs

1. emacs -Q

2. load-library show-wspace

3. open the file 

4. M-x show-ws-toggle-trailing-whitespace

So I think it's most likely a bug/featuref of show-wspace.el

For others trying to duplicate, that library is in the debian package
emacs-goodies-el.