When upgrading a website from Drupal 6 to Drupal 7 I suddenly saw "<!--break-->" as plain text inside the teaser, instead of hinding the following text - what happened?
htmLawed does its job very well - inside HTML tags there are no comments allowed, so you've to configure it to do so:
Under text formats (/admin/config/content/formats) edit the text format which has this problem and add following in the htmLawed configuration field:
, 'comment'=>1
Done :-)
Sources: redhotchilliproject.com and bioinformatics.org