When will the authors be listed alphabetically? Recently, I noticed that some of the papers actually had their author names listed in the alphabetical order. 

Say Mr. Smith contributed more than Mr. Black. Then without noticing the tiny footnote, one will simply take Mr. Black as the first author.

I am wondering whether there exists some certain such scenarios where the authors have to be listed as such. Will it be unfair for the first-author-should-be? 

Update:

I am talking about EECS field. I see most of the papers list the authors according to the contributions. But only occasionally, I see alphabetically-listed authors.
In mathematics authors are almost always listed in alphabetical order. This works, among other reasons, because work groups are organized less by a lab structure so you get more rhetoric of equality between all authors.