The Russo brothers almost quit Marvel over the ending to Captain America: Civil War.
While the Russos were set on the finale that saw Cap and Iron Man battle it out, the Marvel Creative Committee wanted the Avengers to reunite and return to fight the five Super Soldiers discovered at the Siberian base together. Marvel Studios President Kevin Feige sided with the Russos, and the debate went all the way to Disney's Disney's Chief Creative Officer Alan Horn, who also agreed with the filmmakers.
"We reached a point where we said, out loud in a room, 'We're not interested in continuing as directors of this movie if it is going to be about managing politics and a third act," Joe Russo is quoted as saying in the recently released book The Story of Marvel Studios: The Making of the Marvel Cinematic Universe, by Tara Bennett and Paul Terry. (H/T ComicBook.com)
"And I think Kevin was very energized by that," Anthony Russo added. "And also, maybe it gave him some leverage in the situation, because we were so clear about our point of view."
The Russos went on to direct Avengers: Infinity War and Avengers: Endgame, with the latter briefly holding the crown for the highest-grossing movie of all time (until it was dethroned by a theatrical re-release of James Cameron's Avatar).