For more than twenty years now, mindfulness has had the most extensive and positive press coverage imaginable. Nearly all of that press has focused on the health benefits of mindfulness, making it out to be almost a miracle cure for chronic stress, anxiety, depression, insomnia, etc. And the scientific evidence for these benefits is undeniable at this point. This "medical model" of mindfulness had its origin with Jon Kabat-Zinn, a professor of medicine at the University of Massachusetts Medical School. In 1979, Kabat-Zinn began teaching mindfulness as a means of stress reduction at a clinic he operated out of U Mass. His program, later known as Mindfulness Based Stress Reduction (MBSR) slowly gained traction and became the de facto standard protocol for mindfulness research as well as mindfulness therapy in the USA and beyond.
It was, perhaps, inevitable that mindfulness should enter the mainstream in the USA as a mental health and fitness program, rather than a spiritual path, much as hatha yoga did before it. Although hatha yoga was developed in India as a spiritual practice, it is rather more like calisthenics as practiced in America. (My yoga teacher friend says of his students "They do it for their ass".) There are two predominant "spiritual" paradigms in the USA: Christianity and Scientific Materialism. Speaking generally, Christianity is inhospitable to anything that partakes of non-Christian religion and Scientific Materialism is skeptical of anything that seems superstitious or super natural. American culture made it difficult for mindfulness to enter as anything other than a mental health and fitness practice. I don't know whether or not Kabat-Zinn planned this in advance, but the fact is that American mindfulness is now firmly established as "medicine". Bringing mindfulness to the masses in the Western world is a tremendous accomplishment, but doing so in the context of the medical model has a price.
Mindfulness is an ancient technology refined over millennia to accomplish a very specific task. Mindfulness was not designed to help you relax after a stressful day. It was not designed to cure your depression. It was not meant to make you a happier, more productive corporate worker-bee. It might bring about those effects, but they are merely SIDE effects. Mindfulness was designed to unravel the illusion of your separate, static, fixed personal identity and in doing so, free you from suffering. The medical model assumes that you were fine before you got sick and tries to put you back the way you were. Mindfulness assumes you were always sick and the only way for you to get well is to transform consciousness, not putting you "back" the way you were, but creating a new you.
One of the basic principles of medical ethics is usually stated as "first, do no harm". If you are starting from the assumption that the subject was well before they got sick and you want to return them to that original state of wellness, this motto makes sense. But if, instead, you are operating from the premise that the person was sick to begin with and making them well requires, in a sense, destroying the sick "person", that motto makes no sense at all. Because mindfulness is DESIGNED to unravel the cherished illusion of a fixed, separate thing called a "self", it inevitably does "harm" to that sick self. The process of "harming" that illusory self can be accompanied by physical discomfort, the arising of challenging emotions, the emergence of repressed memories, feelings of being disoriented, perceptual distortions, and others less-than-pleasant experiences. With one exception to be discussed shortly, none of these are adverse effects in the same sense as unwanted side effects of drugs. None of them are unwanted harm. Instead they are an expected part of the process of unraveling the illusory sense of self-as-thing. They are harm to the illusion that is causing the illness and therefore integral to the cure. Some of these experiences can be challenging, but a skilled teacher can guide the mindfulness student through them. It is essentially a matter of recycling the reaction through the mindfulness methods. But, as mentioned, there is an exception.
For the vast majority of mindfulness students, the experience of the dissolving of the fixed sense of self will be blissful. But VERY rarely, a person will experience that dissolution as a very unpleasant, even terrifying, depersonalization/derealization event. That is, they experience the loss of the sense of individual identity in a negative way. This is commonly called a "dark night" experience, from the writing of San Juan De La Cruz. That terminology is often used incorrectly for the run-of-the-mill challenges of the practice. But the true Dark Night does rarely occur and the remedy may not simply be to recycle the reaction and the guidance of an experienced teacher is required to lead the person "into the light". There is a risk in mentioning this at all because people may use this very rare possibility as an excuse to just not do the practice and that is a mistake. But it is necessary because it is becoming popular to talk carelessly about supposedly adverse effects of mindfulness.
In summary, during mindfulness practice it is not uncommon for people to experience physical or emotional discomfort, disorientation, the emergence of repressed memories and emotions, etc. However, these are not adverse effects. These experiences do NOT mean something has gone wrong, or that you are doing the practice incorrectly, or that somehow you are unable to reap the benefits of the practice. On the contrary they are actually manifestations of the desired effects of the breaking up of the fixed, separate sense of self-as-thing. Those supposedly adverse experiences are actually a sign that the practice is working as intended. The correct course of action is to simply recycle those reactions, bringing concentration, clarity, and equanimity to the sensory experience. In the extremely rare case of an actual Dark Night experience, it is advisable to consult an experienced teacher for guidance.