Dream Eater has an ability that Surveil 4 when it enters the battlefield. However, this ability is happening while the card is in your hand, before it actually enters the battlefield, as long as it has full mana.
This means that with a full board of three creatures, the option dialogues for Surveil 4 happens first - then, after you Surveil 4 the dialogue box asking to select a creature to replace with Dream Eater occurs (which can be declined). This lets you get a free Surveil 4 every turn as long as the board is full.
Because Dream Eater has flash, there is an opportunity to cast it each time an opponent’s creature attacks. As a result, if your board is full and you disable the Dream Eater in your hand, you can get a free Surveil 4 for each creature your opponent controls that can attack.
//Edited Title - Tombstone
Funny, I’m pretty sure dream eater did not do that originally. I have often used DE as a bounce creature to keep the opponent off by back, but I know there was times that the opponent did have a hexproof creature that couldn’t be touched by DE.
This reminds me of a similar problem with Hydroid Krasis: If you already have 3 creatures in play, and choose not to replace one of them with your Hydroid Krasis, it will pump it’s power and toughness multiple times over successive turns.
Back to Dream Eater; I notice when you play Dream Eater against Kinjalli’s Sunwing, you get to Surveil, but you do not get to bounce.
Surveil is currently triggered when the card is cast, whereas really it should be triggered when the creature comes into play.
Thank you for the reports everyone. I can confirm that the team is aware of this issue with Dream Eater’s Surveil effect.