I taught my cats an expensive habit
What habit would that be?
I guess it's the habit of knocking over milk once they're finished drinking it
Was this intentionally taught to them? Or is just something that they do because they can?
it was intentional from the start. Why? Are there problems with it?
It's weird to teach cats to do something destructive as that especially since you have to clean and buy new milk. You can simply though teach them not to do that or keep milk containers away from them.