Guardians of the Galaxy Vol. 2 is a great movie. It's a hit with both critics and audiences and it won the box office for the second weekend in a row. However, the film is not perfect, and it's all James Gunn's fault. Recently the director of the film admitted that his movie has a minor continuity error. During Stan Lee's requisite cameo in the film, he makes reference to having been a FedEx man, a nod to his cameo in Captain America: Civil War. The problem is that Guardians 2 chronologically takes place before Civil War, and thus Lee should not yet have that story to tell. In Gunn's words...
One thing I found very funny and interesting is the fact that people thought Stan Lee is a Watcher and that all of these cameos are part of him being a Watcher. So, Stan Lee as---probably---a guy who is working for the Watchers was something that I thought was fun for the MCU. And yes, I know I made a mistake. I'll own up to my mistake because Guardians of the Galaxy 2 theoretically happens in 2014 which is before Infinity War. And Stan Lee in the movie says, 'That time I was the Fed Ex guy,' which is what he is in Civil War. I screwed up; I wasn't thinking, But I'm going to say that probably Stan Lee used the guise of a Fed Ex guy more than one time.
James Gunn held a Facebook live chat over the weekend in which he opened the floor to all questions and spoilers were fair game. It was there that he admitted his minor mistake in the film. While the events of most of the Marvel Cinematic Universe have been moving along at a more or less real-time speed, the Guardians of the Galaxy have been doing their own thing. The film opens with a flashback scene to 1980, and then jumps 34 years into the future, which puts the film in 2014, shortly after the events of the first Guardians film.
When we meet Stan Lee in Guardians of the Galaxy Vol. 2 he is surrounded by Watchers and telling them of the time he was a Fed Ex man, but if that happened in 2016, when Captain America: Civil War was released, then it hasn't actually happened yet in the MCU timeline.
James Gunn says that Stan Lee is referring to a different time he was a FedEx man, something that we apparently never saw on screen. We could argue there's anther possibility. If Stan is working with Watchers, there's no way to be sure that Stan Lee is working on the same timeline as everybody else. Perhaps Lee is jumping through time and making these cameos out of order.