1) Doing sautes (a kind of jump) with a low hanging light above you. It will give you a bump on the back of your head.
2) Doing devoloppes when the barr isn't even as long as your leg is. It gives you a sore foot.
I, infact, did these things today while practising ballet. As result, I have a bump on the back of my head (which luckily is healing fast) and a sore foot, (which is still sore).
However, I feel good for practising.