Every Pokémon Trainer's dream item is the Master Ball--a Poké Ball with perfect accuracy that always captures its target on the first try. Or at least, that's how it is in the games; the anime has presented a very different side of the Master Ball, where it failed to capture what should've been an easy target.
In the Pokémon Advanced Generations episode "Whiscash and Ash," Ash arrives at a lake in the Hoenn region only to accidentally drop his badge case in the water, where it's immediately devoured by an enormous Whiscash. Ash tries to battle the Whiscash to no avail, as it's simply so big that his Pokémon can't do much to it. Ash then proceeds to nearly drown chasing Whiscash into the deep and is only saved by the fortunate appearance of a fisherman's Pikachu lure.
The fisherman, introduced as Sullivan, is after the massive Whiscash, known locally by the nickname "Nero." The Whiscash seems to enjoy taunting people, and Sullivan has made it his life's mission to hook this giant fish. Sullivan teaches Ash and his friends how to fish, and they work together to catch Nero. Nero eventually reemerges for the episode's climax, and Sullivan attempts to battle with his Flaafy--a poor choice, since Whiscash is part Ground-type and immune to Electric attacks. Sullivan, however, has been waiting for this day, and produces a Master Ball, tossing it at the Whiscash. Nero simply opens his mouth and swallows the Poké Ball whole, preventing it from opening and capturing him, and leaving the assembled cast stunned. Team Rocket interfere, and Ash's badges ultimately get returned, but the Whiscash gets away and is seen at the episode's end playing with the Master Ball like a toy.
Was Nero an Alpha Pokémon?
Giant-sized Pokémon have appeared in the franchise's core games since this episode aired. Sun and Moon had Totem Pokémon, which were giant-sized and couldn't be caught. Scarlet and Violet's Titan Pokémon also seem comparable, but a better comparison may be Pokémon Legends Arceus's Alphas. Although Titans and Totems are specially chosen in some way, Alpha Pokémon are simply exceptionally large and aggressive members of their species, a description which fits Nero to a tee. Alphas are also usually of very high level, making them difficult to catch. While Nero eating the Master Ball is obviously a joke, this is actually the only appearance of the Master Ball in the anime, giving it an astonishingly bad 0% success rate on screen.
It's understandable why the Pokémon anime might want to avoid Master Balls, as they change the dynamic of capturing by removing the Pokémon's free will. The anime instead emphasizes the bonds that trainers form with Pokémon, so seeing a Pokémon willingly submit to a red Poké Ball is more satisfying. Still, the Master Ball is an important part of the Pokémon franchise, so it at least merited a shout out--even if it took the form of a joke at the item's expense.