The Mystery Unveiled: Walter poisoned Brock in season 4 of Breaking Bad but the truth didn't come out until the following year. The character Brock Cantillo (Ian Posada) was the six-year-old son of Jesse Pinkman's girlfriend, Andrea (Emily Rios). Brock greatly admired Jesse and the two would often hang out and bond. After Jesse and Andrea broke up, he continued to send them money through Saul Goodman. The pair reunited in Breaking Bad season 4, but shortly after, Brock was rushed to the hospital with a serious illness — which turned out to be because Walt poisoned Brock.

At first, Jesse believed that Brock was given the ricin that was intended for villain Gus Fring. He thought that Walt stole the ricin and gave it to Brock as a way to punish Jesse for getting too close to Gus. When confronted, however, Walt gave Jesse the idea that it must have been Gus who used the ricin to harm Brock. Sadly, Jesse was right as Walt caused Brock's illness to get Jesse to help him kill Gus on Breaking Bad using the Lily of the Valley plant, but it isn't clear how Walt poisoned Brock with the berries.

The Sinister Act: Breaking Bad creator Vince Gilligan provided more details about Walt poisoning Brock at San Diego Comic-Con in 2013. The writers of Breaking Bad referred to the incident as Walt becoming the "Evil Juice Box Man," imagining him injecting the poison into Brock's juice box. Off-camera, Walt had somehow planted the juice box in Brock's lunch at school, making sure only he came in contact with it.