Outgrowing the Box

...there was something unusual about the purple-hued horizon. Instead of the sky and the sea meeting in a straight line—as they ought to and always had before—the juncture between them curved, like the edge of an unimaginably big circle. It was such a strange sight, it took Eragon a half-dozen seconds to understand what he was seeing, and when he did, his scalp tingled and he felt as if the breath had been knocked out of him. "The world is round," he whispered. "The sky is hollow and the world is round."

—Inheritance, page 479

That rare moment when a work suddenly reveals itself to be so much bigger, so much vaster in scope, than you ever imagined it would be. When it abruptly shatters the bounds of what you thought it was, in the best way possible. A common audience reaction is, "Oh my god, they went there. They actually went there!" (Where "they" refers to the writers).

This trope exists in the intersection between Crowning Moment of Awesome (only, for an entire work—or, if you prefer, its writers—rather than a specific character) and Wham! Episode. May include Going Cosmic, Growing the Beard, various forms of Tone Shift (in particular, Cerebus Syndrome and Darker and Edgier), or the introduction of a Myth Arc into a work where one wasn't expected. It can also take the form of a Genre Shift or Out-of-Genre Experience, as in the page quote. However, while many of the aforementioned tropes are the result of the writer(s)' preferences changing over time (or of switching to new writers entirely), this trope is usually planned in advance, making it more thematically similar to The Reveal.

Very much a YMMV trope.

Examples of Outgrowing the Box include:

Literature

The Inheritance Cycle (page quote): when Eragon & co. are caught in a massive updraft and sent to higher altitudes than anyone in living memory has ever reached, high enough for them to notice the curvature of their planet, and to see the night sky without light pollution or (a significant amount of) the haze of the atmosphere distorting it and hiding most of the stars. This is an injection of sci-fi into a formerly strictly fantasy series, although it's not a particularly good example of this trope since it goes back to being 100% fantasy after the scene ends.
...And I Show You How Deep the Rabbit Hole Goes: Many readers will find themselves thinking, "Wait, what?" when they see the "ACT TWO" heading. And they have no idea what they're in for.

Video Games

xkcd #1608 - Hoverboard: this one can potentially pull it off twice. 1) Leave the literal box which appears to be the entirety of the game's play area and ignore the warning to return. 2) You can enter cheat codes using the JavaScript console.
In Final Fantasy III after spending the entire game on the floating continent and explored most of it the party leaves it and is put on an entirely different world map.
Castlevania: Symphony of the Night when you reach the end of the castle your map completion percentage approaches 100%. After the boss fight against the apparent Big Bad you are put in another castle that has its own separate map completion percentage, making for a total of 200% completion.