Operating system:
Python version: 3.6.3
Black version: 18.4a3
Does also happen on master: yes

Steps to reproduce:
black --check myproject
1 file would be reformatted, 68 files would be left unchanged.

Run it again:
black --check myproject
All done!
69 files would be left unchanged.

So I cannot reformat the code:
black myproject
All done!
69 files would be left unchanged.

If I remove the cache file, it works and the file is reformatted.