After several hints and oblique references this episode has the first direct that Gotham City and Batman (or at least Bruce Wayne) exist in the Arrowverse. Gotham City has previously been mentioned as existing on Supergirl's Earth-38, (along with repeated references to Batman), but this is the first time that Bruce Wayne has been mentioned on Earth-1.
Quentin's quip to Oliver, "I didn't have a photograph of you in flagrante de-Arrow", is a play on the Latin phrase "in flagrante delicto", which translates to "in blazing offense"; it refers to when a perpetrator is caught in the act during the commission of a crime.
This was the first CW show to name drop Bruce Wayne, hinting at a appearance of Batman later in the season.
Tetrodotoxin is the poison in fugu (pufferfish) that makes it toxic to humans unless prepared a specific way.
This is the first episode in season 6 to not contain a flashback.