I was experimenting with cats added in the snapshot 18w44a and I wanted to see what happens when I spawn a wolf. Nothing happened so I tamed it and it gave me an advancement "A Complete Catalogue", which you should be able to get only if you tamed all cat variants.

What I expected to happen was...:
Nothing, unless there was supposed to happen something else.

What actually happened was...:
I got "A Complete Catalogue" advancement.

Steps to Reproduce
1. Spawn a wolf using spawn egg.
2. Tame the wolf.
3. Notice it gives a player an advancement "A Complete Catalogue".