TYPO3 Core

Thanks for your report, but I cannot reproduce it.
Exactly your example is working fine for me in 6.2.7

I think, you have forgotten the .external option.

Please try

page.includeCSS {
  font-awesome = //maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css
  font-awesome.external = 1
}

See http://docs.typo3.org/typo3cms/TyposcriptReference/Setup/Page/Index.html#includecss-array
.external: If set, there is no file existence check. Useful for inclusion of external files.

Following the docs, I think the behavior was wrong BEFORE 6.2.7

Yes, you are right. This is because it was working without the .external parameter before. The issue can be closed.