That's How Science Works...
From Agatha Christie's Murder on the Links (Emphasis in Original):
“My dear Poirot-never mind all that. We must go to the villa at once. There has been another murder!”
I have frequently been disappointed when fancying that I was giving news of importance to my friend. Either he has known it already or he has dismissed it as irrelevant to the main issue-and in the latter case events have usually proven him justified. But this time I could not complain of missing my effect. Never have I seen a man so flabbergasted. His jaw dropped. All the jauntiness went out of his bearing. He stared at me open-mouthed.
“What is that you say? Another murder? Ah, then, I am all wrong. I have failed. Giraud may mock himself at me-he will have reason!”
“You did not expect it, then?”
“I? Not the least in the world. It demolishes my theory-it ruins everything-it-ah, no!” He stopped dead, thumping himself on the chest. “It is impossible. I cannot be wrong! The facts, taken methodically and in their proper order, admit of only one explanation. I must be right! I am right!”
He interrupted me.
“Wait, my friend. I must be right, therefore this new murder is impossible unless-unless-oh, wait, I implore you. Say no word-”
He was silent for a moment or two, then, resuming his normal manner, he said in a quiet, assured voice.
“The victim is a man of middle age. His body was found in the locked shed near the scene of the crime and had been dead at least forty-eight hours. And it is most probable that he was stabbed in a similar manner to M. Renault, though not necessarily in the back.”
It was my turn to gape-and gape I did. In all my knowledge of Poirot he had never done anything so amazing as this. And, almost inevitably, a doubt crossed my mind.
“Poirot,” I cried, “you're pulling my leg. You've heard all about it already.”
He turned his earnest gaze on me reproachfully.
“Would I do such a thing? I assure you that I have heard nothing whatsoever. Did you not observe the shock your news was to me?”
“But how on Earth could you know all that?”
“I was right then? But I knew it. The little gray cells, my friend, the little gray cells! They told me. Thus, and in no other way, could there have been a second death. Now tell me all. If we go round to the left here, we can take a short cut across the golf links which will bring us to the back of the Villa Genevieve much more quickly.”