Retry Clocks are clocks first appearing in Mario and Luigi: Bowser's Inside Story, and could restart a battle if Bowser or Mario/Luigi reaches 0 HP.
They also reappear in Mario and Luigi: Purple Heroes. The Shroobs hate this device, and continuously go out of their way to break the Shy Bros.'s clocks. A maintenance shop in Toad Town takes care of that problem but the Shy Bros. have to go there quickly as if they lose a battle, the Retry Clocks cannot be used.