What are some examples of situational irony in Part 1 of To Kill A Mockingbird by Harper Lee?

Situational irony occurs when a result or outcome is much different than what was expected. 

In Chapter 6, Scout, Jem, and Dill go to the Radley house at night. When Mr. Radley comes outside and fires his gun, the children run away. Jem's pants get caught on the fence and he has to leave them. He retrieves his pants and in the following chapter he reveals that they had been sewn and left folded across the fence. The children must have thought that if Jem was able to retrieve his pants, he would find them torn and stuck on the fence, but someone had fixed them. This was unexpected.

At the beginning of Chapter 10, Scout discusses how she and Jem think that Atticus is old and feeble: 

Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected upon his abilities and manliness. 

Jem always wants Atticus to play tackle football, but Atticus says he is too old for that. Scout and Jem do not view Atticus as a man of action. They don't feel that he is as manly or as active as other fathers of children their age. However, in this chapter there is a rabid dog on the loose. Heck Tate is poised to shoot the dog, but he asks Atticus to take the shot. Atticus reluctantly agrees and takes the shot. Scout and Jem are mesmerized. They would never have expected that Atticus was capable of such a thing. They would never have imagined that, as Miss Maudie says, "Forgot to tell you the other day that besides playing the Jew’s Harp, Atticus Finch was the deadest shot in Maycomb County in his time."