The following may contain spoilers for the future of the Marvel Cinematic Universe. You have been warned.
Things are getting very busy within the Marvel Cinematic Universe. With so many movies on the horizon, there are chapters being filmed based on events we've never seen, and that's when spoilers start to appear. The Russo Brothers are hard at work filming Avengers: Infinity War, a film that will be out next year. However, a picture of the two on location appears to contain a spoiler for this year's Thor: Ragnarok. Specifically, Mjolnir, Thor's Hammer, which we've seen get destroyed in the Ragnarok trailer, will apparently be back in Infinity War.
The image was posted to the Instagram account for the Russo Brothers and shows a behind the scenes shot of the filming of Avengers: Infinity War. The location, based on commenters who recognize the spot, is apparently Durham Cathedral in London.
Of course, because we're looking so far ahead in Thor's story by looking at Infinity War, the fact is we don't actually know which movie we could be seeing a spoiler for. The fact that Mjolnir will be back isn't too much of a surprise, iconic character equipment, much like iconic characters, doesn't stay dead forever in the world of comic books, so the fact that the hammer will be back won't be an unbelievable surprise for many. However, will Thor have his weapon restored by the end of Thor: Ragnarok or will its return be part of Avengers: Infinity War? At this point, we don't know.
Our guess is that Mjolnir won't be making its return until Avengers: Infinity War, as some concept art for the movie that's been released has shown Thor using different weapons, which would seem to imply that he won't have the hammer for at least part of the film. We could certainly see Thor's plotline in Infinity War dealing with an attempt to reforge the hammer as a weapon to fight Thanos. Though, reforge may be a bit of a misnomer, considering that as far as we can tell, there aren't really going to be any pieces of the thing left when it gets destroyed in Thor's next film.
As I said, the fact that the hammer of Thor will return is probably not a massive spoiler, but now we do apparently have a timeline for exactly when it will happen. Will we see it this November when Thor: Ragnarok is over, or will we be waiting until May of 2018 and Avengers: Infinity War? That seems to be the only question left to answer.