A number of trailers leaked at the show last year, including Deadpool, X-Men: Apocalypse and Warner Bros' Suicide Squad. Teasers that go out ahead of time supposedly hurt the studios in a few different ways: for one, they're usually uploaded in a lower resolution, potentially sullying people's perception of the film. For another, they break an otherwise perfectly planned marketing strategy, which again, studios fear dilute the film's impact with viewers. The counterargument, of course, is that any fan interest should be seen as a positive. If millions watch a leaked trailer, that means millions are now aware and potentially interested in the movie.
A Comic-Con spokesperson told The Verge: "There are a great many things that go into making a great panel presentation. For TV networks and movie studios, sometimes that includes exclusive footage, and while we have been very diligent in trying to prevent footage from being leaked, the truth is today's technology makes any guarantee difficult. We are working with our friends at the different studios and networks in hopes of finding a remedy that will be beneficial to them, us and most importantly the fans."