In 'A Rose for Emily', what is one of the things that wasn't stated clearly throughout the story but the facts reveal it at the end?

In the early part of the story, the narrator spends a bit of time describing the occasion when Miss Emily's house was leaking a foul odor.  No explanation is given for the odor at the time.  The townspeople are unable to get into the house to investigate.  They finally sneak into the yard and sprinkle lime around the house to quell the smell. 

...four men crossed Miss Emily's lawn and slunk about the house like burglars, sniffing along the base of the brickwork and at the cellar openings while one of them performed a regular sowing motion with his hand out of a sack slung from his shoulder. They broke open the cellar door and sprinkled lime there, and in all the outbuildings.... After a week or two the smell went away.

At the end of the story, the reason for the smell is revealed with the discovery of Homer Barron's body.  He had obviously died in the house and rotted there, and during that process, the smell of it had stunk up the town.  This also explained the reason that Miss Emily had bought arsenic.  See this exchange:

"I want some poison," she said.

"Yes, Miss Emily. What kind? For rats and such? I'd recom--"

"I want the best you have. I don't care what kind."

The druggist named several. "They'll kill anything up to an elephant. But what you want is--"

"Arsenic," Miss Emily said. "Is that a good one?" 

The discovery of the body reveals that Emily must have used the poison on Homer.