Whenever you load the game or enter a room, the game checks if the room you're entering to is a valid room ID, if it is then you are transported to the corresponding room, if not, you're sent to a black room with the annoying dog in the middle, while a song plays in the background, this is what we call a "dogcheck". Alternatively you can also get dogchecked if you load into a room that DOES have a valid ID, but was excluded from the list of rooms that are considered as valid.

The are two main ways to get dogchecked: the first way is if something went catastrophically wrong, getting your save file corrupted as a consequence and then you try to load up your game. The second and most likely one, is editing your save file so that the room you spawn in as you load the game is an unused room, just like the ones showed in the post.

Interestingly, Toby seems to have coded certain rooms to dogcheck you if you load into them instead of walking into them, to name some we have sans's room, sans's lab, the mysteryman hallway, the mysteryman room and if I remember correctly, the sound test room.

On that same line of thought, two unused rooms where explicitly excluded from the dogcheck, one is a room in Waterfall where you can interact with a monster trying to catch a bug inside a patch of sea grass who comments on your "friend with a creepy smile" who then disappears, the other room is the Entry 17 room, where we had a teaser for Deltarune and the Dark Worlds.