publications - 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.

Answer

It happens when the authors chose to do so!

Journals do not impose the order of authors in the authors list. They usually give guidelines (or policies) on authorship standards, i.e. who should be co-author and who should not, but they leave the ordering up to the authors themselves.

Now, different fields have different customs. In physics and chemistry, alphabetical ordering is very rare (used for example in consortium publications), while in mathematics alphabetical ordering is more commonly, but not exclusively, used. Unless there is a footnote clarifying the authors’ respective contributions, you cannot definitely tell which system they used.