Currently, if you have something which forgets to std::move():
  OnceClosure closure = base::BindOnce(...);
  closure.Run();

The compile error looks something like this:
../../base/bind_unittest.cc:415:17: error: cannot initialize object parameter of type 'base::internal::RunMixin<base::Callback<int (int), base::internal::CopyMode::MoveOnly, base::internal::RepeatMode::Once> >' with an expression of type 'OnceCallback<int (int)>' (aka 'base::Callback<int (int), base::internal::CopyMode::MoveOnly, base::internal::RepeatMode::Once>')

It would be nice if we could make this more readable: there's an upstream bug with clang that the diagnostic simply isn't helpful (https://llvm.org/bugs/show_bug.cgi?id=31067), but even with that change, I think it's easy to misunderstand this error.