During the DC Nation Olympics Arc, it was completely unintentional for Hades to actually kill Arsenal. The intent by the plot-runner and Hades-mun was just to get him high, removing him from the fight. The fact the Roy player ran with it and made Hades simulate a heroin overdose completely changed the ending, prompting the Sadistic Choice the Greek Gods would force on Donna later.
In Part III of Bart The General, a door bell can be heard ringing in the background when Bart talks about throwing a sponge at a mirror.
Queen Of All Oni: Chapter 14 has a scene of most of the Shadow Hand playing poker, and another of Right comforting Jade after her confrontation with her father. Neither was originally planned, but were added spur of the moment.