Today a question appeared: what to do if an edit is request that actually does not change the content (e.g. setting the sitelink of an item to X when it is already X in the language).
Currently, the API reports an error because the save failed (there is nothing to save, obviously). If you try the same on MediaWiki core, it behaves as if everything went well, but doesn't do a save (i.e. the save does not happen, but the state is the same and thus the software just lets you proceed as if you have made the edit).
(Actually, the front end is currently too smart to let you do that, but a similar situation appears when someone else has changed it to the value you wanted to change it to while you have been viewing the page. Right now we make a conflict. Is this the desired behavior?)
We should be consistent through all API modules, obviously.
Cheers, Denny