So in the Map Editor, you're supposed to be able to spawn mice and mice with cheese using M and Shift M, for a while a lot of people thought the Admins had removed this update, but as Ykr pointed out in the Map Editor Q&A, they exist, they are just no longer visible.

For example, look at this XML



Test out the map and take your mouse and put it on either side, press M/Shift M and you'll see the plank tips over. This is because of the mouse you spawned, however you can't see it, and that's the bug. It used to be you could see the mice, they aren't supposed to look like this.