When Winn hypothesizes that there might be an alternate Earth where they are all evil, Barry says he's been there and that "it sucks." The world to which he is referring is Earth 2, where Barry's friends Cisco Ramon, Caitlin Snow and Ronnie Raymond were all supervillains.
Kara's impulsive attempt to defeat the villains without a plan reflects Barry's similar attitude early in his own career. Barry's insistence on developing a plan and checking the perimeter before rushing into the fight are strategies he originally learned from Oliver Queen, a.k.a. the Green Arrow. Here, he passes those lessons on to Kara.
When Kara describes how her exposure to red kryptonite briefly turned her evil, Barry says that he understands because the same thing happened to him. The incident he is referring to is his first battle with the Roy G. Bivolo, in which Bivolo used his emotion manipulation powers to release Barry's anger. Co-incidentally, this was a crossover episode with "Arrow", where Oliver demonstrated the importance of searching new environments before rushing in. Barry also tries to teach this lesson to Kara.
Barry objects to Cat Grant's naming his alter-ego "The Blur" instead of "The Flash." The former name is a nod to Smallville (2001) Clark Kent is known as "The Blur" during its last three seasons.
This episode aired the day before Flash Back (2016), that which precedes it chronologically speaking. This meant that viewers of The Flash (2014) who watched it when it aired were out of sync with Barry's personal timeline.