15SPIDER-MAN HOMECOMING TIMELINE
Well, perhaps "mystery" isn't exactly the right word, since it's more of a slip-up than anything.
Spider-Man being in the MCU is a bit strange when you think about it. Spider-Man: Homecoming was a Sony film, but it took place in the MCU and the film made a point to show just how connected it was to the rest of the films. There was the fact that
Iron Man was a major supporting character, all the easter eggs in the cargo ship and the fact that the film's villain was essentially created from the aftermath of the Battle of New York. And speaking of the climax of The Avengers, the film, it is also responsible for one of Spider-Man: Homecoming's biggest mysteries, its timeline.
Regardless, we are aware, through various sources including an official MCU timeline infographic released by Marvel, that that The Avengers and Captain: America: Civil War take place in the years they were released, 2012 and 2016 respectively. Yet, in the opening for Homecoming, we find Adrian Toomes and his crew cleaning up after the Battle of New York, followed by an "8 years later" title card. But, that would mean that Homecoming took place in 2020, despite the fact that it's supposed to take place a short time after the events of Civil War. Marvel has admitted this was a mistake, but it still leaves the question of when Homecoming actually took place.