Relayed from BA.
Broken: 2.79b to 2.92 master
Short description of error
Using the eyedropper of the Parent property, if used on the same object, leads to creation of a custom property 'parent' on that object, with a reference to it. This can result in data that is never orphaned.
Exact steps for others to reproduce the error
In the default scene, select the cube, open Relations tab in properties, click the eyedropper on the Parent field and click on the cube. And error message will indicate 'Failed to set value'. However, user count for the object will increase to 2, and a custom property 'parent' will be created for the cube.