The plot line for the upcoming Avengers sequels has been shrouded in mystery, and for good reason. There are Marvel films still left to release before Avengers: Infinity War gets shown to the public in May 2018, all of which will help contextualize the universe.
That makes protecting it one of the highest priorities for movie studios. Zoe Saldana may have let the cat out of the bag on future sequels and plot details while she was out to promote Guardians of the Galaxy Vol. 2. In the video up top, you can see her talk briefly about filming for Infinity War, but in doing so she accidentally lets slip that the Infinity War sequel will be centered around the word "gauntlet."
For fans of the extended universe, it's a major tell on where the series is heading next. Infinity War is about a battle undertaken by the Avengers and the Guardians of the Galaxy crew to stop Thanos, whose quest to assemble the Infinity Stones is an ongoing plot line in the series. The implication of "going back to film Gauntlet," as Saldana puts it, is Thanos will succeed in assembling the Stones by the end of Infinity War, and fans will be left waiting until the next Avengers movie to see what his reign of terror looks like.
Marvel is desperate to throw people off the scent, and director James Gunn claims Saldana did not in fact name the sequel:
It's entirely possible Saldana slipped up or was simply mistaken, but if true, Marvel may have to alter their plans moving forward. Marvel Studios President Kevin Feige recently said the title for Avengers 4 hasn't been revealed because it contains a spoiler. Telling people the potential title and where the plot will end in the third Avengers movie is a spoiler alright, so expect Saldana's reveal to be heavily scrutinized in the months to follow.