It's okay to have subs, but they do throw off your groove. You learn to expect a certain type of combination with a certain type of port de bras to a certain type of music and then BAM! Suddenly you are trying to figure out what the hell is going on and you haven't even made it past tendus at the barre, yet. And you spend the next hour+ sort of shaking your head and guessing and/or flailing around hoping you hit a ballet position of some sort by blind luck.
When we would universally botch the hell out of an entire combination this teacher would give us another shot at it (or torture us by making us repeat something we obviously can't figure out? You be the judge.) which meant we ended up doing quite a few things over and over again. After watching us glide gracelessly across the floor for the requisite bourées en pointe she stopped us with a question. "So. Who here loves doing bourées?" and everyone just sort of stood there, listening to the crickets chirp (honestly, I enjoy them more than most things en pointe, but after two hours of class there are few things in the world that I like). SO! "Okay, you are doing them again. AND I want to see those happy faces and softened arms. You're performing, guys! You can do this! Smile!"
I suppose we smiled. Or perhaps we just grimaced in a convincing approximation.
I think smiling is probably the hardest thing about ballet.