This cautionary stuff has come up here more than a few times. Gerard is correct in that engaging methodologies that heighten energy can tend to amplify pathologies.
However excessive emphasis on how 'dangerous' everything is only heightens self-consciousness and discourages people from trying.
One aspect of both qigong and pranayama is purification - the release of toxins and obstructions of various kinds. The process tends to go from physical to psychological.
and spiritual, hence liberating karmic imprints from previous lives which if not dealt under close supervision of someone experienced in the subject could lead to... I leave this open because I don't know as I don't have enough experience on the subject. A really experienced medical Taoist I know of told me about this stuff.
Practices also can simply increase energy - if such methods are employed mechanically - without some degree of objectivity and intelligent self-observation - but with considerable force - then there can be negative effects as the energy is simply channelled into existing patterns.
For example: At a workshop we were practicing Kapalabhati Pranayama. One woman complained that the practice was making her feel horrible. As it happens whe was suffering from Lime Disease which was in it's active phase at that time. Kapalabhati enhances the immune system but engaging it during an active illness is counterproductive and creates more stress on the organism. In her case a less forceful - soothing pranayam was indicated.
In Yoga we have a term 'vinyasa krama' - which means to proceed intelligently in steps. So - start at the begining, build capacity for practice and awareness together and gradually. Be consistent, attentive and observant from the begining.
Placing emphasis on States, spiritually speaking, MAY be useful insofar as they can highlight what they are not. Emphasizing the development of powers and so forth is also not the point. To the extent such orientations are true they may be employed in the context of comprehensive traditions and adapted to the needs of individual practitioners with a clear sense of purpose and perspective.
Before someone launches off on some great list of potential personal catastrophies - or attempts to 'explain it all' it terms of some fragementary shard -
let me conclude with this:
Vinyasa Krama - start at the begining - proceed gradually - ask for help when you need it.
Agree