Full Idea
Black holes have a temperature, and hence entropy. ...But if a black hole are just an extreme scrunching of smooth space-time, it should have no substructure, and thus no entropy. This is probably the most obvious incompleteness of general relativity.
Gist of Idea
Black holes have entropy, but general relativity says they are unstructured, and lack entropy
Source
New Scientist writers (New Scientist articles [2013], 2015.11.07)
Book Reference
-: 'New Scientist magazine' [ 2013], p.31