promises-aplus/promises-spec — pull request #134 by ForbesLindesay (2013-07-08 11:16:50+00:00)
Title: Encourage warning messages for `onFulfilled` or `onRejected` being promises

This is probably the most common error I see from beginners using promises on stack overflow (e.g. http://stackoverflow.com/questions/17517593/more-complex-deferred-promise-usage-and-chains-just-arent-clicking-would-love/17525172). They call the function, rather than passing the function to `.then`. Since I can't see why anyone would intentionally pass a promise to a `.then` handler and these sorts of functions almost always return promises, I think this could be a big win for helping people get started faster.

P.S. the PR is just there as a discussion point, I'm happy to re-word/move it as appropriate.