I'm not normally a big fan of media where you can't fix your mistakes easily. Watercolours have to be the most precious of all of the colours: if you even look at your brush the wrong way, you'll have made a mark you can't erase. Some people think (an art teacher of mine included) that it's the mistakes that make the art special. I say that if you inadvertently get blue paint on your forearm and then make a big blue splotch on your paper riiiiight as you were putting the finishing touches on a fashion illustration that you were doing for the second time already because you didn't really like how the first one turned out, then that is indeed special. Very, very special.
That being said, however, with practice comes...not perfection, but at least more confidence. We have been doing portraits (from photographs) for the past two classes and although I initially balked at all of the steps involved (drawing the face onto a clean sheet of plain paper, fixing the proportions, outlining the shadows, covering the back with graphite, tracing onto the watercolour paper, fixing the lines, and THEN starting to paint - and I didn't even talk about preparing the watercolour paper) I see now that of course those steps are there to help you make fewer mistakes. The more patience you have with the structure and prep work for media like this means that your final product will be, inevitably, more satisfying. Though it won't necessarily guard against getting paint on your arm.
That can happen to anyone.