Yoga is in the news again, though no longer as a panacea for all the world(s)' problems, or as a benign souvenir from the so-called mystical "East," but as an easy way to injure one's body. If you are a yoga practitioner who believes that your practice (or your guru's practice) has its roots in the Yoga Sutras of Patanjali (or as some yoga teachers call him, "Pot and Jolly") then be warned: there are dangers when yoga is practiced incorrectly that are far greater than mere bodily injury. In fact, if one fosters the wrong cognitive habits while doing yoga then the danger is rebirth in this mundane reality (samsara). Thinking the wrong thoughts while doing a downward facing dog pose (adho mukha shvanasana) (or while observing someone else do a downward facing dog pose!) will delay release from the cycle of birth and rebirth, known in the Yoga Sutras as the preeminent state of kaivalya (isolation).
You should be.
According to Patanjali, the root cause of suffering and rebirth is an incorrect cognitive habit, namely the misidentification of consciousness (purusha) with the material world (prakriti). The body, one's body, of course, is included in this. In fact, it is only when one isolates, separates and distinguishes consciousness from materials stuff, from one's body, that one can achieve liberation. This goal of reaching and maintaining the right cognitive habit is attainable by following the practices enjoined in the ashtanga (eight limbs) of yoga, only one of which is asana (posture).
So for Patanjali the postures (asana) of yoga are a mere means to an end, and not an end in and of themselves. The body is used, then, as an instrument for attaining the right cognitive habit, divorcing and detaching consciousness (purusha) from the material world (prakriti), from the body. In the same way that some poetry serves to move the reader outside of language, the body can be used to transcend the body.
It is thus ironic that so many do yoga as a mere physical practice, or as a way to meet other body-aware people wearing spandex, or expensive and politically correct, but nonetheless sexy, yoga outfits. Practicing yoga this way leads to rebirth and suffering. In fact, obsession with one's body (and with the person's on the mat in front of you!) conflicts fundamentally with what is prescribed by Patanjali.
So, while it is true that yoga can wreck your body, Patanjali thought that obsession with your body in yoga (and with the bodies of others) can wreck your mind, reinforce your undesirable cognitive habits, and can sabotage your chances for breaking out of the cycle of birth and rebirth!