Game rules stability is the property where, if you change the game code, you don't break how cards behave. To help ensure this, code design on Artifact is heavily test-driven. If we add a mechanic we can auto-check that we didn't break a bunch of card interactions. 
New cards are implemented through tests even before the card is playable on the client. When we find a rules bug, we also write a test for that case so we'll know if we break it again in the future. We like self-documenting tests, so we end up with some crazy test names.
The tests themselves check only a single idea at a time and are designed to be as simple as the idea allows. This test ensures that if a Melee Creep hits a tower that as Assault Ladders the tower takes 4 damage instead of 2.
In addition to card tests, there are tests for basic functionality like core combat rules, timer rules, phase progression, shopping, etc. Some of the tests shown above use old names for cards instead of their current names - Assassin's Shadow used to be called Berzerker Champion.
This system doesn't guarantee that we never ship card bugs or break old cards, but it really helps and if we do break a card we can add a test and prevent that kind of bug from happening again. This is a standard engineering practice, but one we thought was interesting to share.
