WordPress 4.0 added a grid view to the Media Library. Clicking a grid image opens the standard modal, revealing the custom meta details.
If an ACF field group’s location has been assigned to Attachments, a JavaScript error is thrown when the modal opens.
error: Uncaught ReferenceError: acf is not defined
Here’s a screenshot for reference.

I want to confirm this issue. Any time I add a field to attachments I get this JS error.
Getting the same error on my user profile mod…
Can’t submit the form either, but that’s probably a consequence rather than a cause!
I’m also getting the same error with custom fields on Attachment forms (not just grid view).
I have ACF 5 and WordPress 4.0.1.