There are tons of exercise inventions, toys and apparatus that are illegal because someone hurt themselves and sued. I suspect we are missing out on some brilliant training equipment and other fun stuff because no manufacturer was will to take the risk.
Allow students to wear socks in a dance class and you are at risk of negligence. Neck rolls were out for a while. I remember when they were a staple of African Dance classes. Neck rolls seem to be back but with lots of warnings to 'go very slowly.'
OK, that was just my preamble. The thing that drives me crazy is the knees must not go past the toes injunction. I hear this all the time. This must have come from a lawyer.
There are two parts to it. The first is basic physical education. If a weighted knee rolls inward and the foot turns outward simultaneously, the Anterior Cruciate Ligament is in danger of breaking. Everyone needs to know this. Just telling people to keep their knee over their toe is not enough information. Students need to understand what they are training to avoid. But a knee really can move in a complete circle around the foot as long as forces are not putting that ligament at risk. In fact it is a good idea to train this way because it teaches the student to keep their whole foot on the ground, there-by avoiding rolling over on the ankle and many other possible strains.
If the knee goes forward past the toes as the heel is coming upward a great deal of strain can be put on the patella and it's various attachments both directly above the knee and inside it. This is why squats were quasi illegal for a time and some ballet teachers just abandoned the grand-plie. But there are important exceptions. If the whole foot remains on the ground with some weight maintained in the heel and the Achilles tendon has time to reach it's full length-- then it's perfectly safe to let the knee go forward of the toe. In fact, I consider it a necessity for students who want to learn to utilize the power of the legs, or want to learn effortless kicking techniques, or for basic shaolin stances like bow and arrow stance.
With the knee forward of the toe and the Achilles tendon fully extended the heel can even come off the ground. This is used in Bagua Zhang's so called "lower basin" training, and in Daoyin dragon walk. These two are advanced techniques and need to be introduced over time, but they are safe.
Some of the old masters were brutal. And it is probably true that people got badly hurt every once in a while. I'm happy to leave that in the past. But there is also a kind of rough confidence about the body which comes from tens of thousands of hours of practice. To the untrained eye that confidence may look dangerous or risky when in fact it is a gift and a treasure.
(Facebook readers: there are more images if you click "original sourse" below)