Factual error: The entire NYC subway platform/subway knife fight between John Wick and Cassian appears to have been filmed in the Port Authority Trans-Hudson (PATH) system that goes between NYC and NJ rather than the Mass Transit Authority (MTA) subway system (you can actually see the PATH logo on the exterior of the train cars). The PATH does not go to any of the subway stations announced on the public address system during the knife fight. (01:18:45 - 01:22:50)
Factual error: After John Wick's house blows up, he takes his dog for a long walk into the Continental in Manhattan. He lives in New Jersey and walks into Manhattan over the Brooklyn Bridge which is on the complete other side of Manhattan, crossing over into Brooklyn and not New Jersey. (00:26:45)
Factual error: The party in honour of Gianna, Santino's sister, is held at the Terme di Caracalla (Baths of Caracalla). That site belongs to the city of Rome, and would never be used to host a private party, let alone a party in honour of a major Mob figure. Moreover, the place where John and Gianna meet while she's about to take a bath is the Horti Sallustiani (Gardens of Sallust), which are some 5 km away from the place where the concert is being held.
Answer: String Quartet in A Major, Op. 55, No. 1, by Joseph Haydn.