Episode 501: [spoiler](#s "How does Brock get poisoned by the Lily of the Valley?")

Walt doesn't see Brock at the time he gets poisoned. I'd originally thought that Saul had slipped something to Brock, since he was delivering money to Andrea regularly, and there was a scene showing Saul being friendly to the kid, but in the first episode of season five, Saul states that he had no idea Brock would end up in the hospital.