Looks like some Pokémon Sword and Shield players using Surprise Trade are getting more than they bargained for, as hacked Pokémon have been crashing games - and even disabling online features.
First made public by Pokémon modder Kurt on Twitter, the bug occurs when a "maliciously-crafted" Pokémon is sent via the game's Surprise Trade feature (basically Pokémon roulette). While receiving one of the malicious Pokémon doesn't corrupt save data, it crashes the game when the player attempts to access online features: effectively disabling online functionality in that particular Sword and Shield save.
According to player accounts, the bug can also occur when a hacked Pokémon is sent during a normal link trade - but this only happens once, and the offending Pokémon can simply be released afterwards.