I just created an item with a sitelink. When I tried to add statements to the item, I got an error saying that the sitelink was already in use by another item (not that I was trying to change any sitelinks). It seems that when I tried to create a new itemWhen creating Items and Properties on Special:NewItem and Special:NewProperty respectively, two items with the same sitelink were createdit's possible to submit the form twice.
The items are https://www.wikidata.org/wiki/Q27067644 and https://www.wikidata.org/wiki/Q27067645Submitting the form twice will lead to two entities being created or the entity being created, but the user still gets an error message (because it can't be created twice).
I started with https://www.wikidata.org/wiki/Special:NewItem?site=hakwiki&page=%E5%88%86%E9%A1%9E%3A%E9%81%93%E6%95%99&lang=hak&label=%E5%88%86%E9%A1%9E%3A%E9%81%93%E6%95%99
As far as I remembern order to fix this, I clicked in one of the input fields and pressed enter to submit the form.