"Prior to 1.9, an ajax call that expected a return data type of JSON or JSONP would consider a return value of an empty string to be a success case, but return a null to the success handler or promise. As of 1.9, an empty string returned for JSON data is considered to be malformed JSON (because it is); this will now throw an error."

That one is gonna cause some headaches, I've abused this behavior for years, as recently as yesterday.