An important instance of dramatic irony occurs in Chapter 9, when Poirot interrupts a conversation with Sheppard, Flora, and Major Blunt to reach into the pond after something shiny. He comes up with nothing; after Flora and Major Blunt leave, Sheppard laments that Poirot got his suit muddy for nothing:
“And all for nothing, too,” I said sympathetically. “I wonder what it was in the pond?”
“Would you like to see?” asked Poirot.
I stared at him. He nodded.
“My good friend,” he said gently and reproachfully, “Hercule Poirot does not run the risk of disarranging his costume without being sure of attaining his object. To do so would be ridiculous and absurd. I am never ridiculous.”
As it turns out, Poirot did in fact retrieve something from the pond. He shows Sheppard a woman's wedding ring with an engraving: "From R., March 13th." He tells Sheppard that he carefully concealed the ring in front of the others by dropping it into his other hand before showing the empty palm of the hand he had used to reach into the pond. Just as Sheppard likely only tells his patients what they need to know, Poirot says, he concealed the ring because Flora and Major Blunt were better off not knowing about it, at least at this point.
Poirot's choice to show the ring to Sheppard ostensibly demonstrates that Poirot trusts the doctor more than he trusts Flora and Blunt. On a closer reading, though, he may not be as trusting as he first seems. Poirot simply doesn't have anything to lose by showing Sheppard the ring, which can't be said of Flora. The ring is a woman's wedding ring, and Flora is one of the only women around. What's more, some have suspected her of secretly marrying Ralph Paton, whose name begins with an R. The ring might very well belong to her. Then again, the "R." in the inscription could be someone else's initial—Roger, perhaps, or Raymond. What is fairly clear is that Sheppard is neither the ring's owner nor the person whose initial is inscribed on the ring. It is safe for Poirot to show him the ring, and doing so conveniently makes Sheppard believe Poirot trusts him implicitly.
In Chapter 27, Dr. Sheppard at last admits that he is the killer and explains how and why he did it. Even as he takes responsibility, he notes the situational irony that many other people seemed to implicate themselves in some way over the course of the investigation:
I wish I could have known beforehand that Flora was going to say she’d seen her uncle alive at a quarter to ten. That puzzled me more than I can say. In fact, all through the case there have been things that puzzled me hopelessly. Everyone seems to have taken a hand.
Sheppard was especially "puzzled" when Flora claimed to have gone into her uncle's study at a time when Sheppard knew the man to be dead. Clearly she was lying, but why? Sheppard knew that he worked alone to commit the murder, so he could not imagine why Flora would lie and place herself in a more guilty position. Sheppard notes that Flora was not alone in muddying the case: "Everyone seems to have taken a hand," he writes, "puzzling" him and even covering up what really happened.
Sheppard's surprise at this situational irony is ultimately his downfall. If he had understood that innocent people often behave as if they are guilty, he may not have relied so heavily on framing Ralph Paton. Poirot, on the other hand, insists over and over that even people innocent of the crime in question always have secrets and often look very bad. This attitude pushes Poirot to investigate every possible suspect, especially the people who look the least guilty. Sheppard does not initially come off as a viable or motivated suspect, and that is precisely what makes Poirot suspect that he might have something to do with the murder.
