Shouldn't a user interface be as consistent as possible?
I believe when a user uses an interface, he experiences certain behavior from the interface, and tries to couple those experiences to the rest of the interface as well.
As an example (and the reason why I posted this question) take the accept answer icons on Programmers.SE.
When looking at these icons, I wonder what the checkmark means. I notice the up and downvote icons are either filled in or not. The filled in icon is filled in since I upvoted. I can't couple this behavior to the checkmark, since it doesn't have an outline. What is the current state of the checkmark?
Here I accepted an answer, and notice the checkmark 'lights up'. When comparing to an answer which I didn't accept, I can clearly see the difference that I accepted this answer, and not the other. There is a problem however, when I want to see whether I accepted an answer, and it is the only one there.
So, I believe it would be a big improvement for the accept answer icon to also be outlined. Of course I posted this on Programmers.SE meta as well.
Obviously I agree with the highest upvoted answer, but I was hoping more on a 'scientific' UX answer, perhaps some terminology, or influential articles discussing this topic.
So to rephrase the question: Shouldn't a user interface be as consistent as possible?