When I add a statement to a Wikidata item I must 'save' the statement (and any qualifiers) and then as a separate editing action click 'add reference'.
If I try to add a reference to the new statement in the same edit action, the system locks-up, and I must refresh the page - the 'cancel' option doesn't even work at that point.

I am using Mac OSX 10.10 , and this occurs both on Chrome and Safari for any wikidata item/statement.