Health problems. I'm not totally clear on the details as I haven't so far seen anything elaborating on it. As far as I remember my teacher just says one can become abnormal.
I've heard a few stories myself about people flipping out on meditation retreats, although there was no Tantra involved. Some Christians also believe that meditation makes you susceptible to demonic possession. And there are all sorts of mental and emotional problems one may face in meditation, although the worst I've seen was a few people crying or be frightened of some experiences. I'm not saying that one can't do harm with practices, but it sounds more like empty threats. As I see it, the worst thing that can happen when one is without an experienced guide is believing in things that are not there. Alas, without detailed and reliable information this is not a topic that can be explored.
Astus all and ALL,
I just Googled "secret teachings" and over 4,500,000 hits were found. For "secret teachings in the Vajrayana", over 113,000 hits were found. Perhaps there are not so many secrets anymore.
People who are going to flip out, and who are harmful to themselves and others, exist in all communities, at all times. I have a great story about rescuing a guy who flipped out during my first ChNNR retreat in 1982. People present all sorts of mental health issues.
Whenever you do anything in this age, someone will misunderstand, you and mess it up. This just goes with the territory of DOING / PRESENTING anything, Vajrayana included. You have to expect this, allow for it and work with this.
In previous ages it was possible to restrict secret teachings to a limited audience. With the web, nothing is secret. Maybe the historical reasons for secrecy no longer apply.
Recognizing this, perhaps it is more responsible to explain, and put in context, all secret teaching and then expect common sense and peer pressure to control and limit harmful, or crazy making, behavior.
If you are teaching something, then you want to explain the proper cautions and possible dangers involved. When pursuing a spiritual path, it is really helpful to have a Teacher to guide you to avoid wasting time and to avoid any harmful path.
Certainly, seeking Teachings from only a recognized Master is one way to protect yourself, but then you are limiting yourself to the Teachings of that Master. Maybe this is a good thing.
If your goal is realization, then one path will do fine. If your goal is to "understand" everything then you need to collect all knowledge / Teachings and then have a way of cataloging it and working with it.
Perhaps these are different goals and different needs/rules for secrecy apply. An academic doing a cross cultural comparison has different rules than a Vajrayana initiate in retreat.
It is a complicated issue.