Try it without making the file writable.|
This happens, e.g., if a file is edited as root by a user using vim,
which doesn't care whether it's writable or not, since it's root
doing the editing, and then by a user using Emacs, which does
care because it detects VC and thus makes the file read-only in
Emacs. The user's first impulse will be to type C-x v v, which will
overwrite the previous editor's changes.
The problem goes away when vc-mistrust-permissions is set to true.
I would argue that C-x v v should always check the status of a file
with vc-mistrust-permissions set to true (or the logical equivalent)
before overwriting its contents and potentially losing data.