We found this with the maven-metadata-plugin, where it's certainly more problematic than with EnvInject. If you save empty password fields (which with maven-metadata-plugin is "normal"), then e.g. Chrome will augment the POST with a saved Jenkins account password (and the user will not necessarily notice this).
But since the cure is easy and unintrusive, is it really important how often accidents might happen? I doubt you'll ever want autofill in these fields.