Revealing mistake: When Dyatlov goes outside the control room to check on the situation after the explosion, his collar and button strip on both his shirt and coat change positions. When he's in the room, it's facing his right, but when he leaves the room and see graphite on the ground below him, it's facing his left.
Other mistake: The reactor at Chernobyl exploded at 1:23 AM. Near the beginning of the episode, we can see that Vasily Ignatenko was at home with his wife, sleeping in his underwear, when they were both startled by the explosion. Later on, we see him in full gear, entering the power plant on a fire truck. We're informed on-screen that it's 1:30 AM. That means that Vasily had to get dressed, get to the fire station, put on his gear, get in the fire truck and get to the plant (~3 km away) in 7 minutes. Impossible. In fact, the first firefighters arrived at Chernobyl at 1:45 AM.
Other mistake: Legasov is told that a 30km zone will be evacuated, shown as an ill-founded cabinet decision. Legasov himself proposes a 200km zone. At the end of the episode, Legasov appeals to Gorbachev (General Secretary) and asks him to evacuate 2600 square kilometers. It's easy to check that this is a 29km radius circle, nothing like the 200km he proposed.
The Happiness of All Mankind - S1-E4
Factual error: When Bachu talks to Pavel during their lunch, two bottles of vodka are visible. Top of the bottle should read: RUSSKAYA, however, it's misspelled and reads "RUSSKAVA." Other issue with those bottles - in the center it should read in Russian "Русская Водка", however, although "Водка" spelled correctly, the "Русская" is misspelled and ends on "ерская", which is incorrect. (00:30:26)
The Happiness of All Mankind - S1-E4
Continuity mistake: When Legasov proposes to use Bio-robots, Scherbina pours 2 glasses of vodka. One glass is about 80% full, another 60%. However, when glasses are placed on the table, they appear to be both filled to 80%. (00:39:27)
