In the Pokémon universe, a Master Ball guarantees the capture of a Pokémon, but they are very rare, so it’s best to think carefully before using a Master Ball. Unfortunately, the Pokémon GO player in question was a bit too hasty in using their Master Ball.
The player encountered a (Galarian) Moltres and decided to use their Master Ball to guarantee the capture of the legendary Pokémon. This, in itself, wasn’t a bad idea, but the disappointment came swiftly. It turned out that it wasn’t a real Moltres but a Zorua, a Pokémon that can transform into other Pokémon, much like Ditto.
Certainly, it was an embarrassing situation: wasting a Master Ball and not gaining a legendary Pokémon. However, the player could have realized something was amiss. Other Pokémon GO players quickly pointed out online that the Moltres was wearing a Buddy Ribbon. This was essentially a giveaway that it was a Zorua, as Zorua appears as one of your Buddy Pokémon from the previous month.: Oh no! There goes the Masterball