One of the subplots in
the season 4 finale of Breaking Bad is that Brock, the 8yo son of Jesse's season 4 girlfriend, Andrea, is rushed to the hospital with a mysterious critical condition. She calls Jesse, who rushes over to support her and see Brock. (Jesse has a soft spot in his heart for boys who are growing up in tough situations.) "At first I thought it was a flu, but then it didn't get better," she explains. The docs don't know what it is, either. They move Brock to the ICU.
After being refused entrance to the ICU, Jesse goes outside for a smoke. In his pack of smokes he sees that the special cigarette he kept in there, the one poisoned with ricin as part of an older plot between Walter and him to kill drug lord Gus Fring, is gone. Jesse thinks about how Brock's symptoms are similar to how Walt explained Gus would die once poisoned with ricin. He freaks out, thinking that Brock might've nicked one of his cigs to try and picked the deadly one. Jesse freaks out and runs back inside to find Andrea. He whispers to her in conspiratorial tones, "Ricin. Rice-in. Tell the doctors that word. They'll know what to do."
Next Jesse confronts Walter. The two are already estranged. Jesse accuses Walt of deliberately giving that cigarette to Brock. Walt objects, pointing out that 1) he had no opportunity to steal Jesse's pack of cigarettes, remove the poisoned one, and give it to Brock; and 2) What would be his (Walt's) motive for killing Brock? There's no self-defense situation there, no "He knows too much" story, and no get-ahead-by-killing-him benefit. He's an innocent kid.
Walt argues to Jesse that if anyone connived to put that cigarette in Brock's hands, it must've been Gus. Walt explains how Gus would have motive, means, and opportunity. Walt's explanation is fishy- it's a long shot for us viewers to believe what he's selling- but Jesse, in his frantic state, buys it. Or at least he buys enough of it to stop suspecting Walt- and to aide
Walt's new plot to kill Gus with a bomb.
A few other twists happen in this subplot, but the resolution is that Brock was not poisoned with ricin. The doctors figure out that he was sickened after eating berries from a plant called Lily of the Valley, which grows locally. The berries taste sweet but are poisonous.
The final scene of the season is the camera panning through Walt's back yard. One of the potted plants there is identified by a plastic tag, the kind you see when buying plants at any nursery or lawn store. Lily of the Valley.
Does this mean that Walt poisoned Brock?? Some fan sites state matter-of-factly that he did and it's proof. I don't see it, though. Like Walt said when arguing with Jesse, he had neither opportunity nor motive. There literally wasn't a spot in the timeline, as portrayed in this episode and the one before it, for Walt to give those berries to Brock. If the matter-of-factness of some fans' statement comes from the showrunner telling them, "Yeah, Walt did it," then that's some serious bullshit writing.