Steven Universe - Be Nice To Lapis

That means you, Jasper

I personally understand Lapis situation and kinda get why she did it, but is still the worse decision someone has taken in this show. And yes, I'm counting "let's run into this corner!". Malachite is something that harms literally (and I'm using literally correctly here) everyone involved, and benefits no one at all.
And the ocean thing. Good thing the show has "no endor holocaust" because in real life, even if she took only a small section of the ocean and then returned it, it would have caused irreparable massive damage to the ecosystem that could have also affected the ecosystem in other areas of the ocean (for beginners, it was several hours out of water that should easily have killed all the coral and rooted algae that many animals depend on). If she took a big part of the ocean, it could easily have kickstarted a mass extinction.