Pokemon Shining Diamond and Shining Pearl bring the challenging Gym Leaders of the Sinnoh region back to life. This includes Candice and her ice gym in Snowpoint City, and if you're not careful, you could get completely stuck inside.
As shown by Pokemon analyst @eclipse_tt on Twitter, it is possible to get trapped in the snowballs in the center of the Snowpoint Gym. You can't put them aside once you're down there, forcing yourself to start over from a previous save. Fortunately, avoiding the fall isn't too difficult - just be very careful around the stairs in the center of the room. Make sure you have a backup save even before entering the Gym, as an extra precaution. You definitely won't lose all your hard-earned progress.
The Eclipse thread also describes a speedrunning technique: you can slide and slide past all those snowballs and hills and go all the way to Candice herself. Attempting to do this could cause the aforementioned softlock, so be careful. If you've already conquered the Snowpoint Gym, you can always regain the Gym Leader here as well.
If you want your ice type Pokemon, you can evolve Eevee into Glaceon. This is one of the many special evolutions of Brilliant Diamond and Shining Pearl. The list includes modifications like Clefairy in Clefable, Nosepass in Probopass, and more.