In my opinion the most dangerous asanas so far are the asymmetric asanas. One of them is eka pada sirsasana, one leg behind the head. It's difficult to let develop this pose slowly, because either the leg is in front of the head or behind and this makes a huge difference. I still feel a certain discomfort on the right side of my back. I blame this pose for it. It's true: Before this asana two twists shall neutralize the body after the deep back bending asana. But then comes without much preparation this challenging leg behind the head pose. I usually prepare this pose and I add an exercise: I stretch the leg to the sky (or ceiling) holding it with my hand (right leg, left hand), leg is behind my shoulder.
It's important to practice with patience. :)
Some variations how to prepare the pose can be found in the book "The complete book of Vinyasa Yoga" by Srivatsa Ramaswami.