In the spawn area of the Zombies map "Der Eisendrache", there is a complicated Easter egg first discovered in 2019 that can be executed. When you spawn in, do not open any doors and allow the Zombies to break down the five barriers in the spawn area and enter the map. Once most or all of the Zombies are in the map, kill them for points. This should be done by the start of Round 2 and it is important to leave one or two Zombies alive and roaming the map so that you do not progress to Round 3. Once Round 2 starts, you need to go to every barrier and attach exactly two boards to each one, then go purchase the RK5 pistol. Next, you need to go back to every barrier and add two more boards to each for a total of four boards on each barrier, then go purchase the Shieva assault rifle, replacing the RK5. After that, you need to go back to every barrier one more time to attach one more board to each for a total of five boards on each barrier, then go and purchase the RK5 again replacing the Shieva. From the moment you purchase the RK5 again, you need to race to the fenced-off door to the east of where you first spawned in under five seconds. If you get there in time, you can walk out onto the deck and find a heart-shaped arch spinning on a platform in mid-air. Inside that arch is a picture of developer Drew Marlowe and his family.
In the multiplayer map NUK3TOWN, if you shoot off the arms on all of the mannequins, an alarm will sound in the distance. All of the mannequins in the map will then pursue you when your back is turned and will freeze in place when you face them. This is an homage to the Weeping Angels from the Doctor Who series, at which an enemy in the show that pursue people in the same manner.