"Sherlock," John said, stopping as the detective, still with his back toward the room, held up a hand, then dropped it. John waited, watchful and ready. They stood like that, in silence, for a few minutes. When Sherlock still didn't move, John went quietly to his chair and seated himself, eyes on his friend. I know I don't understand, Sherlock, but I'll be here, as long as it takes. A half hour passed in silence. Finally Sherlock spoke, his back still to John.
"You have no idea, John, how many times I have replayed that night in my mind," he said. "Every possible angle, every observation, every deduction, I have chased down and confirmed. Every bit of it indicates that I was right. It all hangs together. I just went over it again now. I could not have been wrong. Yet," he gestured listlessly toward the envelope without looking at it. "I was wrong."
"Couldn't it be a mistake? Anderson, for instance," John began.
"Was on still back at the flat, packing up," Sherlock interrupted, turning to face John. "And you'll remember that I allowed no one into the classroom that night until Lestrade and the evidence crew arrived with their collection supplies. Lestrade came in with me, to protect the crime scene, but I collected the pills myself. You can be sure that I marked each bag very, very carefully."
John thought. "What about the handling and the processing, then?"
"I had a word with Lestrade. I emphasized that the chain of custody was essential in this case. He agreed and took charge of the evidence personally. I think he was actually embarrassed about the raid on the flat and wanted to make it up to me. He's seen to it that it has been handled with the highest possible protocol, with no chance of the pills being mixed up accidentally. I asked for a Class A analysis, so there is no possibility of a lab error. The samples have each been checked with gas-liquid chromatography, mass spectrometry, various reagents, scanning electron microscope, and with radiological equipment. Biological components, had there been any, would have undergone suitable testing depending on the type. The tests confirm one another. I expressly instructed that I not be informed of preliminary results. I wanted to be absolutely sure, to hear only the final conclusion, once. I've been waiting for it for months." He turned back to the window.
"Well," John said, "Could the cabbie have switched the bottles after you decided which was the safe one?"
Sherlock closed his eyes and said, patiently, "Yes, John, and of course he did. There was a moment when I had gotten up from the table and was pretending that I was going to walk out. I had my back turned to the cabbie." He paused.
"And?"
"As expected, I heard the cabbie switch the bottles. I also saw him do it in the reflection from the door window. It was a forced move in our little game. I made my final determination, made my final choice. Then I returned to the table and snatched up the bottle I wanted. He was quite intrigued by my selection, as I recall. Not too long after, we were ready to, as he put it, 'take our medicine'."
"Good thing you didn't," John said. "I'm glad you were wrong. You would have died."
"No, John, I wouldn't have," Sherlock said softly.
John looked sharply at his friend's back. "What?"
"The pill I was holding was the safe one. I meant to take the other."