What are the mysterious facts that readers are presented with which are not explained until the end in detective story The Murder of Roger Ackroyd (Agatha Christie) and crime novel Strong Poison (Dorothy L. Sayers)?

The Murder of Roger Ackroyd has one of the most famous surprise twists of all time in a detective novel. The 1926 story is told by Dr. Sheppard in the first person as if he were a detached bystander recording events as the great detective Hercule Poirot solves a murder mystery. In fact, we learn at the end that Dr. Sheppard is the murderer! He manages to present the facts of the story accurately by glossing over certain details of how he performed the murder. As he leaves after the murder, he only notes he checked to see "if there was anything I had left undone." In another example, he has set up a dictaphone (what we would call a tape recorder today) of Dr. Ackroyd talking to play in Roger Ackroyd's study after he is dead, so it will look like Dr. Sheppard was not on the scene when Ackroyd died, and he also arranges to have himself on hand to be at the murder scene right away (as a doctor) so that he can remove the dictaphone. However, rather than writing down in his story that he removed the dictaphone to cover up his murder, he simply writes "I did what little had to be done." He sticks close to Poirot and imagines writing of the failure of Poirot to crack the case: instead, Poirot sees through him. But the key point is that the person who seems to be the story's reliable narrator is really the murderer! (I remember how stunning I found that when I first read this book.)
In Strong Poison, Sayers's heroine Harriet Vane is accused of poisoning her ex-boyfriend Phillip Boyes. She appears to be the only person who could possibly have murdered him with arsenic the evening he died. He had been to a dinner party, but there all the food was shared by at least two people: if any food item had been poisoned, someone else would have been sickened and died too, but nobody did. After the party, Boyes drops in on Harriet, who gives him a cup of coffee. Since he dies soon afterward, the assumption is that Harriet, an angry ex-lover, put the arsenic in the coffee.
However, we find out at the end that one of the people at the dinner party did poison Boyes. It was Norman Urquhart, who built up an immunity to arsenic by taking it over time in small doses. He shares an arsenic-laced omelet with Boyes at the party, but because Urquhart has an arsenic immunity, he does not get sick. Boyes does react to the arsenic in the normal way and dies.