As The Flash's and Arrow's creators are the same, it's normal we see the spin off making references to the other series. On the last Flash's episode, "The Sound and the Fury", we saw it happening twice - have you got those? -, and of course we're not talking about obviously things, as the love story between the hero and the detective's daughter.
Lower than the next reference, Harrison Wells came to the public and said in front of cameras that he had failed Central City's people. That's what Oliver Queen says when he meets the ones who are poisoning his city - in this case, Starling City.
When Hartley Rathaway, Wells' prodige, a.k.a Pied Piper in the comics, told us how he discovered where from The Flash were, by retracing Barry's path to the crimes scenes, we remembered of an episode from the third season of Arrow.
In "Draw Back Your Bow", Cupid did the same thing to discovered that the Team Arrow's cav where located under Verdant's club. Same strategy, and they both worked as planned.
Cupid is coming back to Arrow with the Suicide Squad.
Guys, this is my first article, so, take it easy!