We already know that Pokemon Sword and Shield users are sending Pokemon through Surprise Trade that aren't even part of the game yet, but now some bad actors are taking things a step further.
A handful of hackers have decided to use their powers for evil by sending out glitched Pokemon data through Surprise Trade. If you get one of these Pokemon, your game will crash when you try to access online features via the Y-COM. This prevents online interactions, and the only solution is a patch from Pokemon Co..
The one bit of good news is that this won't corrupt your save data. Still, it's not something you want to experience, so it might be best to stay away from Surprise Trade until a patch is released.