When Caitlin examines Barry after the chemical attack, the wound on his side is shaped like the state of Virginia. Actor Grant Gustin has a tattoo there shaped like his home state, Virginia.
When Iris asks about Caitlin's "cousin," she mentions two cities - Midway and Coast City. In the DC comics storylines, Coast City is the hometown of Hal Jordan (The Green Lantern), and Midway City is the hometown of the Doom Patrol as well as Hawkman and Hawkgirl.
After Caitlin Snow mentions that Ronnie and Dr. Stein's brain waves are in perfect sync and that it's "next to impossible," Barry notes that impossible is "just another Tuesday for us." This is a direct reference by the writers to The Flash (2014)''s air date on the CW on Tuesday nights.
Cisco says that they "have a guy locked up in [their] basement who can turn himself into poison gas. And that was just Week 3." This is a nod to Kyle Nimbus (a.k.a. The Mist) in Things You Can't Outrun (2014).