In one sequence in the background you can see the Berolina-Bulidung at the Alexanderplatz. On the roof you can see the sign of the bank company "Bankgesellschaft Berlin". In the time between the fall of Berlin-wall (Nov.1989) and Germany's reunion (October 1990) there was no Bankgesellschaft Berlin situated in East-Berlin. The Company bought the Berolina Building in 1993.
When Alex goes to the supermarket to buy his mom's favorite pickles, he asks an employee where to find them. The employee is tagging cases of Pepsi. The Pepsi logo on the cans is the modern day logo, not the one in the early-'90s.
When Dennis and Alex arrive at their firm and their colleagues are watching a football match in 1990, the worker who is opening a bottle of champagne at the end is wearing a t-shirt from the 1994 soccer championship.
Denis wears a "digital rain"-style T-shirt in 1989 because he has developed the idea himself and has come up with an idea for a film exactly like The Matrix, which he describes in a deleted scene (the letters are not identical to the Matrix scheme.) The joke is that the idea originated in East Germany; compare the claim in one of Denis's fake news shows that the Coca-Cola formula was invented there. It also ties in to the film's main theme of keeping people in a simulated reality.