A lot of people in Yoga classes treat it as a competition and try to perform at or above the level of the people on neighbouring mats. Not a good thing to do if the person next to you is an ex-gymnast with 10 years of Yoga experience.

Everyones bodies are different and the goal is to do the exercises as 'correctly' as possible for your current abilities. This isn't taught enough (or corrected enough thanks to very high class numbers or teachers paid a pittance that they couldn't care less about productive practice) and it causes people, most of whom have very little body awareness, to push themselves into a position they shouldn't be in.

Think stretching your obliques in the triangle pose is easy? Chances are, your hips aren't in line and you are just curving your ribs to give you the extra bend. Can you actually feel it in your obliques? Can't keep that back leg straight in warrior 2? Shorten the distance between your feet. If your back leg isn't straight, you're not doing the exercise correctly and putting strain on the wrong parts of your body. Knee turning in? Front leg way too far forward over the foot? Too far back behind the ankle? etc. The amount of chair poses I have seen where the bend in the knees is deep but the back is completley curved blows my mind. What do they think they are doing? Balance poses find their way around this somewhat as it's hard to balance correctly if the basis of the pose is wrong.

People think they can achieve so much with a 1 hour class every week because yoga doesn't look like it should be difficult. They don't seem to understand that, as with any sport, for those who are really good, it is a way of life. Do you think you could learn to play the guitar just by playing in 1 hour lesson every week? Particularly one where the teacher doesn't correct your mistakes?

TL:DR - Too many people do the exercises wrong and too many teachers don't correct.