When using Special:SetSiteLink and providing some bad infomation such as https://test.wikidata.org/wiki/Special:SetSiteLink/P123/enwiki/foo (Which fails as '"P123" is not a valid item id. Site links can only be set for items.') the information is not retained in the text boxes in the form.
When posting such data and returning an error the data should still be available in the relevant text boxes!
i.e. P123 in ID, enwiki in Site id and foo in Site Link