I have ended up with an unusual regression scenario on emacs master, which I can reproduce only with the combination of M-x shell-command-on-region and the par CLI utility.
I don't see the issue on emacs 26.1.
The issue is that when I select a region of text and do `M-| par`, I get this error:
ERROR in parse_header: buffer was ';; This buffer is for te'
error: client: reader got error Success
I got that error by launching emacs -Q (master branch), selecting the text in the scratch buffer and doing `M-| par`. Interestingly that error doesn't happen on running `od', or `cat' or any other command via M-|.
I contacted the par author Adam Costello, and he had this to say:
> > I grepped Emacs source and par source and none contain any part of
> > that error string.
> Emacs versions earlier than 24 have a lib-src/fakemail.c that contains a
> parse_header() function. Could that be involved?
But I don't know .. this is a very recent regression .. some time after 26.1 release.