I'm struggling to figure out how you would write lab instructions in neither 1st person or passive voice. Would you just credit all actions to an anonymous experimenter, described in third person?
That's fair. For psychology methods it's easier to avoid both, at times, since you can write "participants viewed...", etc.. For other types of methods, especially where the object becomes implicitly understood, or would be redundant to state, I can see where passive voice might be useful. For instructions, you can just leave off the implicit "you should", EG "put X in Y". The tradeoff between passive and first person is important, and which is appropriate likely depends on the circumstance. They're both discouraged, but not banned, I'd imagine because poor writing often uses one or the other too often.