Oscarus

In the Meloetta Quest, you have a mandatory fight against Cass - one of the few times you fight a dev outside of Dev Room. 

And her teams (which are 3-on-3) differ on... whether you pet Lumi's Glaceon in Glass Workstation or not. Which might be one of the least expected ways for split. But it makes... a HUGE difference.

If you pet Lumi's Glaceon (which I did in my playthrough; whenever I have the opportunity, I pet cute Pokemon; Glaceon's very cute, all Eeveelutions are - if I could, I would have all of them + Eevee as pets irl), she'll have A-Ninetales, Cinccino and Mega Gardevoir. Pretty solid and standard-fare team, right?

But, if you don't pet Glaceon at least once... Cass will throw at you U-Turning Special Sweeper Naganadel, Dusk Wings Necrozma with Ultranecrozium Z, and Choice Band Rayquaza with Dragon Ascent... oh my god! (at least they didn't have PULSE2...)

Who would've expect that a simple gesture could change one event from doable... to pure agony. That's why I LOVE Reborn - EVERY decision matters. Even the smallest ones. I can't wait for Rejuvenation's V14 when you'll be given the question "Do you prefer Pepsi or Coca-Cola?" and depending on what you pick, someone will battle you with either decently strong team... or Dimensional Rift Ferrothorn, Angel of Death Gardevoir, Dufaux and Puppet Master in 4-on-4 Double Battle on Dimensional Field.