Fourth episode Carlos Valdes (Cisco) is absent from.
When Nora asks Eobard to teach her how use his own speed force that he created he warns her her by saying that's it's "a little negative"a reference to the Negative Speed Force which in the comics was created by Eobard Thawne as a negative version to the actual speed force, an extra-dimensional field of negative energy that eats away at the regular speed force like a cancer and can consume any speedster connected to it by touch.
The title of the episode is named after a story arc/ team from the comics called "Mud Pack" made of all 4 Clayfaces at the time.
Thawne talking about his Negative Speed Force to Nora echoes how he spoke of the Speed Force during the season one finale "Fast Enough".
In the West house, a box of pizza is shown with the logo, "Milgrom and McFarlane", a reference to Todd McFarlane the creator of Spawn and also worked on alot of Marvel and DC titles and who popularized Venom, and Al Milgrom was one of the creators of Firestorm with Jerry Conway.