Covering for people's mistakes is not the job of the healer. Having a good healer can help pull a bad group through their mistakes, but if you fail because people are taking avoidable damage and you just can't heal through it, that's not your fault.

If you notice people making mistakes, say something and give them pointers on how to be better. You can try to fix people being stupid, but you sure as hell shouldn't be trying to heal it.