As a teacher, these are a few of my least favorite things to see:
--Heels lifting off the floor during demi-plie at the barre. Keep them down, down, down! Not only do you shorten your Achilles tendon by keeping them lifted but you also will develop a bad habit that you will take with you to the center. You can't get height or build strength if you do not put your heels down.
--Toes lifting off the floor during rond de jambe and tendu. Tendu means "stretch" and you're not stretching if you allow the toes to come off the floor. Instead, point the toes and feel the legs lengthen all the way to the top of the thigh. By consciously feeling the floor with the baby toe, you will distribute the weight properly which will result in better positioning for releves and pirouettes.
--Hands flinging during allonge. To allonge means to draw out, stretch, lengthen the line. In particular, when we talk about the allonge line for an arm, we mean to rotate the arm up and back, away from the shoulder. Allonge does not simply take place in the wrists and hands. Start with the shoulder lifting up and back and then lengthen the arm away from it, back and up on the ecarte line, all the way to the hands.
--First arabesque arm that reaches higher than the head. The hand should be directly in line with the nose, no higher, and the arm straight at the elbow (unless you are doing Cecchetti in which the arm is often slightly bent at the elbow). But no matter what style you are studying, do NOT lift that arm above your head. It's an affectation that has repercussions through the rest of your body, destroying the line.
Next time I'll talk about things I do like seeing from students!