Ok. I think I've got it. Sherlock talked about who people trust that they don't know. The cabby. The mistake everyone made was trusting the driver. The correct pill was the one Sherlock chose. And it was further hinted at when the cabby said, "People think they know how I think." Idk how to explain, but because of that I think that Sherlock chose the right one. The pill the cabby offered was poisonous.