"Has improper value for" is a predefined property that tracks input errors for irregular value annotations and is provided by Semantic MediaWiki. In most cases it is caused by a type mismatch or a value restriction. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
This is the page of special property Has improper value for. It lists all pages using this special property.
The property connects a page where some invalid input was given to the property to which this value was assigned. One possibility is that an editor copied and pasted a name that seemed to have a harmless space in front but actually had an invisible "tab".