What is an example of situational irony in Chapter 1 of To Kill a Mockingbird?

Situational irony can be defined as the occurrence of an event when a completely different outcome was expected. An example would be where an individual purchases a firearm to protect himself against crime and is killed with the same weapon when he is attacked.
In chapter one of To Kill a Mockingbird, the best example would be related to Jem, Dill, and Scout's fears regarding the Radley house and its infamous resident, Boo. The three were quite apprehensive about the dangers of going near the house and were paranoid about the idea of entering the premises. All sorts of pernicious rumors existed that Boo Radley was evil and that he would harm those who made contact with him. It was imperative, therefore, that no one should, in any way, seek him out or offend him, for they would become victims of his malevolence.
Dill challenged Jem to enter the Radley place. He eventually relented, as a matter of pride, to accede to his request since he had to prove he was not a coward. The result of his action, though, did not achieve the expected outcome, as the following quote illustrates:

Jem threw open the gate and sped to the side of the house, slapped it with his palm and ran back past us, not waiting to see if his foray was successful. Dill and I followed on his heels.
Safely on our porch, panting and out of breath, we looked back. The old house was the same, droopy and sick, but as we stared down the street we thought we saw an inside shutter move.

The only response they received was the slight movement of a shutter, not the conflagration they expected.

Flick. A tiny, almost invisible movement, and the house was still.