What corrections can you make on galley proofs? When I receive galley proofs of a paper, I look at possible errors introduced by the copy-editing team. But while I proofread the article, there are sometimes small mistakes I'd like to fix, which were not introduced by them (i.e. they were already present in the accepted version of the manuscript).

Usually, the proofs are accompanied by instructions saying that extensive changes should not be introduced at that time, and any such changes would have to be approved by the editor (hence, I suppose, delaying publication). However, the limit is not very clear to me. What is considered extensive changes? In particular, what do you think of the following items (from my experience):


Slight changes in wording, to improve clarity
Updating a citation, because an “in press” or “ASAP” article now has page numbers
Adding an important (but not crucial) citation one had missed, in the introduction
Adding a citation to a paper that has been published since the manuscript was submitted; possibly adding a short sentence to the text




What I have done so far is change everything that I think should be changed to improve the paper (including all the above items), and let the typesetter decide whether he wanted to send it back to the editor. I never received any complaint or comment on my changes, which could indicate that it was the correct course of action.
A galley proof should primarily be proofed for typos or mistakes made in the type-setting process. It is not the time to change phrasing, exchange figures, add or remove blocks of text or anything else substantial. 

Adding references might seem like a useful addition but the problem is that it would then be possible to add references without the knowledge of the editors and of course the reviewers and hence possibly make changes that could have affected reviews etc. The case of adding anew paper is a similar problem since it may change the paper in ways that the editor and reviewers have not seen and therefore ok'd. If such changes are wanted (they are probably seldom needed), it would be best to at least check with the editor if that would be appropriate. In short, no changes should be made that alters the content of the paper. All such details should have been checked and if necessary corrected before submitting the final revised manuscript for copy-editing and proof production. Any corrections called for by the copy-editor and editors after submitting the final version is of course to be made.

Updating of references are usually also acceptable, to, for example, add the publication year (from e.g. "in press") or adding doi, page numbers etc. if these were not known at the time the final version was submitted. 

The proofing stage is not a stage where many changes should be made. The manuscript that is submitted for proofing should be considered the last chance to make any substantial changes. What many do not realize is that all changes done after a type-set proof has been produced may cost the journal money, apart from the extra time and trouble it causes. So, as an editor, I often have the feeling authors just send in their final manuscript without checking figures and text properly and then waiting for the proof to make final changes but is, in fact, an abuse of the system.

Regarding your last paragraph, the type-setter is usually not a scientist and has no idea of what changes might mean so to think that the type-setter would act as some form of intermediate editor is not right. In cases where type-setting is done in-house it might be a professional doing the type-setting but I would still say this is not the way to handle the type-setting/proofing stage.

So, anyone, make sure the final submitted manuscript is checked, complete and correct.