When Pele is injured during the game, he rolls around in pain, and the top his head touches the chalk line. The large white patch on his head is clearly seen. In the next scene, it is gone.
When the German player (Baumann) takes the penalty kick, his hair is dry in a scene a few seconds prior to the kick, then it's wet with sweat when he places the ball and then it's dry again after he scores.
During the final football match several of the flags feature a counter clockwise swastika meaning it is not the symbol of National Socialism but the sign of the Buddha.
The escape tunnel comes up under a pool. As the tunnelers break through, air bubbles burst forth - but since the tunnel is open at the other end, water would just flow down the hole, pushing the air along (rather like a domestic bathtub where bubbles don't come up from the drain while the water is running out).
During the match an injured POW player is taken off by stretcher and another player puts on a shirt to replace him. Substitutes were not introduced in this way until 1953; until that point, a "substitute" was a player who stood in for another who had failed to turn up for the game and players were not replaced once the match was underway.
When Stalone is in Paris subway, he looks at a list of stations that must appear on a map. We see the station "Guy Moquet". The 1940s station name was "Marcadet - Balagny". Guy Moquet was a communist resistant executed by the German in 1941. The station was named after him only in 1946.
The vast majority of the extras in the soccer stadium have hairstyles and wear clothes associated with the late 1970s and early 1980s (long hair, flares and wide-collared shirts etc.).
The microphone used by the radio announcer at the soccer match is an RCA model 77B, a model not introduced until the late-1940s.
At one point in the final match a close shot of a little girl in the crowd is shown, behind her a man has on a digital watch.
At the start of the final match, the commentator says: "To ensure fair play and good sportsmanship, the German organizers have picked a neutral to referee the match". The word "neutral" is clearly dubbed in, and does not match the mouth movements of the actor.
The team refuses to escape during the halftime, and return to the stadium to play the second half of the game. The two allied officers in the bleachers see this and are shocked, looking very disappointed, probably thinking something went wrong and the escape couldn't be arranged. However, there is no reason for them to be disappointed, since they could have no idea that the escape was arranged for the halftime break. Even the players were told just moments before the game.
Around the 61th min. after Hatch escaped, when the German soldier comes to report the number of prisoners, he speaks in Hungarian, but not in German.
In the resistance café, hatch finishes his drink and asks for "un autre". In French this would be a request to take his drink away and replace it rather than requesting a second drink. He should have said "encore". While Hatch is not a native French speaker and in normal circumstances this would not really matter, small details like this are giveaways the Gestapo would look for to identify escaped PoWs and allied spies.