Would having the display errors setting turned on in the following admin page admin/config/development/logging cause Varnish to generate cache misses?
Messages are stored in the session (errors, warnings and info messages). If anything is in the session, a session cookie is set in the browser. If the session cookie is set in the browser, Varnish is normally bypassed.
Note, the name of the Drupal session cookie is
SESS followed by a long hexadecimal string.