An overlay that lasted until the next accepted evaluation was a durable
annotation on the source, which is not what it is: it is feedback about the
action that just failed, and the moment you move, type or evaluate it is
describing a program state nobody is in any more.
pre-command-hook rather than post-command-hook, which fires at the end of
the failing command and would take the overlay down before redisplay ever
drew it. Buffer-local and installed only while an overlay exists, so a
session of twenty buffers does not end up running this on every keystroke
in all of them.
Joseph Ferano
c1a612abfb