We use events heavily in my community. A subset of our events are public and have the Going / Interested / Not Going selection buttons. In the following image, all buttons are grey.
Numerous of my users have gotten themselves into situations where they thought they clicked Going but they don’t show on the list of attendees.
The root cause of this seems to be that the tick embedded in the Going button is not particularly bold. When you press the button the first time, the tick does illuminate green 
but you have to look closely to be sure. When you click a second time, the tick returns to its default grey. In the following image, the Going button is green
Contrast that will the Interested button. It is much easier to see the status change with each click.
