I empathize with you on the useless cow or screw up conundrum. I feel it pretty frequently on my rotations. Usually, I end up the useless cow because not doing anything means someone (a patient or a medical practitioner) has to wait a little longer, but taking action and being wrong could really screw something up. But the whole situation sucks (and I wish people would realize that sometimes waiting for instruction is the more responsible thing to do, even if it's less impressive.)