- Ass Pull: Although Siobhan Smythe's supervillain origin story tying into Celtic mysticism is similar to how it was in the comics, nothing of the sort had ever been vaguely hinted at since the character was introduced in the show. Moreover, there had been no hints whatsoever that magic is a thing that exists in this universe, as all the other superheroes and villains have been either aliens or science-based.
- Broken Base: The episode itself—a great idea that will hopefully lead to more crossovers and build a bigger fanbase for both shows, or a lackluster unfocused mess that just confirms the feelings of the detractors of both shows.
- Crossover Ship: Barry and Kara have been shipped since way back when the Crossover was announced.
- Hilarious in Hindsight:
- Ezra Miller (who plays Barry in the DC Extended Universe) once said in an interview that he didn't see himself as the movie Flash but as the Flash of another dimension. Given the dimension-hopping Barry is doing, it seems Ezra's comments can be taken as Canon.
- This Crossover Episode reunites Grant Gustin with fellow Glee alumna Melissa Benoist, and they're up against a smug, Jerkass "big snob" named "S. Smythe". Grant's character in Glee is named "Sebastian Smythe".
- Thanks to the inability to properly line up the crossover episode airings between the two networks, the episode of Arrow that aired the same week as this one also contains a reference to Mariah Carey.
- Cat snarking that Barry, Kara, James and Winn look like the "attractive, yet non-threatening, racially diverse cast of a CW show" becomes this since Supergirl's second season moved from CBS to the CW.
- Ho Yay:
- Just Here for Godzilla: The ratings went up 30% for this episode—looks like Barry Allen has a lot of fans.
- Romantic Plot Tumor: Many believe that choosing this episode to resolve Kara and James's Will They or Won't They? is a mistake since a quarter of the scenes and dialogue are devoted to it, instead of Siobhan's transition into Silver Banshee, her teamup with Livewire, and/or the visiting Hero of Another Story.
- They Wasted a Perfectly Good Character: Alex and J'onn, two of the show's most developed characters other than Kara herself, are not in the episode to interact with Barry.
- They Wasted a Perfectly Good Plot: Despite the nature of the Crossover and its relevance in the entire DC mythos, The Multiverse is not given more emphasis and focus. Supergirl in particular has one of the most well-known (probably for different reasons) Alternate Selves of existing heroes.
YMMV / Supergirl (2015) S1E18 "Worlds Finest"