Are there things that one shouldn't be mindful of? Just to move on from this answer in the question Is practising mindfulness ever a bad idea?

Are there particular things that it is a bad idea to be mindful of? Are there thoughts, feelings, emotions etc.. that one would be better advised not to apply mindfulness to? What are they? Or can every aspect of human existence be included productively in mindfulness.
Yes there are. Being mindful of "signs" that turn the mind to anger (or lust etc.) is unwholesome. E.g. someone irritates you and you insist on attending (upaṭṭhāna) to that particular experience (a product of your mind!), instead of focusing on something more positive.

Mindfulness in and of itself is a neutral factor of feeding, the question is what you feed.