Well according to this article there is a dark side to meditation that everyone should be aware of and some people should never embark on the practice of meditation, because people can get stuck in the process in some dark spaces, according to some psychiatrist, who label these experiences as anxiety, or mania or depression or mental illness in general.
I am afraid I don't buy most of what psychiatrists say, as they know very little about spiritual experiences and the mystical path. Our society, so ingrained in materialism does not understand at all that sometimes the person seemingly spiralling downward is actually on route to a greater experience of the light...it is a process that can be facilitated by a real teacher and they are too far too few and in-between..the shamanic journey is similar...before one becomes a healer one is led through suffering and torture...the wounded self...This article talks about people stuck in some parts of their journey because no one was around to help them through who understood that process...to know God or Higher self first requires tests and obviously the people mentioned here did not get appropriate support to pass these tests...it is not meditation that is the problem but lack of understanding and acceptance and appreciation by our society of mystics in general.