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.

  jik