[USUM Spoilers I guess.]
Imagine, if you will, you're a Pokemon trainer in Alola, taking on the Pokemon League as the final step in your island trial. You handily defeat the four members of the elite four using your team of Primarina, Lycanroc Dusk, Ampharos, Tsareena, Alolan Marowak, and Dusk Mane Necrozma. You reach the throne, but someone stops you from actually taking your seat. Your friend you've been taking the trial alongside has appeared, ready to fight for the championship. So you send out your Pokemon, and he in turn sends his out. The battle keeps going until you're down to one Pokemon each, your Dusk Mane Necrozma and his Crabominable. You activate your Necrozma's Ultra Burst, and because you want to finish it with your strongest attack, which requires that you're in Ultra Burst for at least 1 turn, you start off with a simple Night Slash. Not very effective, but who cares, you didn't plan on taking him out here, even though you probably could have with your Photon Geyser. Except now that the Ultra Burst is active, your Necrozma's now a Psychic/Dragon instead of the Psychic/Steel it was. And your opponent uses Ice Hammer, which not only takes advantage of the new typing Necrozma got, but also Crabominable's Ice typing and high Attack stat, and OHKOs your Necrozma. You literally lost because you wanted to use your strongest attack.