Here's a little mystery for us, but Ultrasun/Ultramoon postgame spoilers ahead because it's about Team Rainbow Rocket.
We good? Cool. So when you face Lysandre he first makes you do the Xerosic gambit from X/Y where you press one of two buttons, one that activates the Ultimate Weapon and the other that activates the warp pad needed to proceed through the Rainbow Rocket castle.
No matter which button you choose, it's revealed to activate the Ultimate Weapon and now you've got to quickly beat Lysandre to push the other button to deactivate the weapon and power up the warp pad.
The mystery is, do you think Lysandre is lying?
Other than his word on the matter, there's nothing to indicate that the world is in any trouble (well, at least more trouble than Team Rainbow Rocket taking over the Aether Foundation and unleashing a lot of holes to ultra space). So are we really to believe this guy was going to blow the world up, again?
I personally have my doubts for a couple of reasons.
1. With one exception in Ghetsis, Giovanni seems to have all the former evil team leaders under his leash, and destroying the world doesn't seem like something in his best interest. And although all the leaders seems to be keen on following their agendas to do their plots all over again, I get the feeling that Giovanni is just humoring them for his own ends.
2. Is the Ultimate Weapon even an option? If this game's timeline follows one from X/Y, that would mean the weapon got wrecked after the conclusion of the Team Flare plot. And even if it didn't, from the reaction of the Alola reporters Team RR just popped up, which suggests they haven't made a pitstop in Kalos to pick up the Ultimate Weapon. That is, unless the pokemon-world is just that clueless and out of touch on international politics.
While it's possible that Lysandre took the weapon from his world with him, I'll remind you that the thing is gigantic! About the size of all small town or at least most of the Aether Foundation.
So where in the world are they hiding that thing? Another option is it's in the "void between worlds" of Ultraspace, which could work I guess. But from Lysandre's comments, it sounds like Giovanni plucked him from his world shortly after the Ultimate Weapon fired in an instant. This makes it seem unlikely that Lysandre took more than what he had on him at the time.
3. With save scumming you can tell that both buttons do the same thing regardless of order, which makes me think they're both just duds for a mind-game Lysandre is putting you through.
I mean, unless this is some Schrodinger's Cat scenario from
Virtue's Last Reward. If so, Gamefreak is really going down the rabbit hole on this alternate reality bender they've been on for 4 years.
A plausible explanation is it's not the buttons but the amount: press 1 and you release the weapon, pressing 2 buttons stops the weapon and powers the warp pad.
4. Can we just say that the whole 2-button scenario is pretty stupid to begin with? The only way it makes sense as a firing mechanism is if you expect someone to stop you just to pull this gambit on them. Why else would you complicate your control panel like that?
And even if you did want a mind game, if the result doesn't matter then why not just use one button and lie that it's the warp pad power?
And other than the fact this happened before (although that was Xerosic, not Lysandre), why does Lysandre expect company? Both Maxie, Archie and even Ghetsis are surprised by your presence, so even if the other grunts gave an intruder alert that gives you, what, 15 minutes to prep this shell-game gambit? Nobody could have known you were coming to storm the castle.
It all adds up that the simple solution is Lysandre's full of it and the Ultimate Weapon was never there. He just really likes head games I guess.
Counter point:
-1. Lysandre's attitude. Ambitions aside Lysandre seems decently sane, and his reactions to your button choice suggest that he truly believes all of this. So either he's just an actor with a good poker face...
You know, because Lysandre was known for being subtle.
...or he meant every word. So the only way the Ultimate Weapon could have been a trick is if Giovanni lied to Lysandre.