2) Acute stress - Certain events, such as a severe car accident or a violent rape, can create such a spike in cortisol that it overwhelms a child's ability to cope. Such acute events can lead to PTSD.
3) Social stress - We have never known social stress to be a good thing, under any circumstance. Not only does it create a spike in cortisol levels that is higher and stays on the brain longer than any other type of stress, but it can linger and endure for months, years, or even a lifetime. Social stress is easily recalled and can be just as potent from recalling the memory of it as it was when it originally occurred. It's meant as a strong warning signal that keeps us in line with the group and is meant to prevent banishment (something that in our evolutionary past meant certain death) but in modern day society, it serves little purpose other than to create severe hurt.
4 ) Uncomforted stress - No stress is beneficial unless it gets resolved, because unless the child learns to deal with it and grow from it, nothing is being accomplished.