[the_why_of_y]

The following code would do the right thing; see if you can spot the difference, and think about whether you would catch that in code review (g++/clang++/visualc++ won't warn about it).

  {
    mutex_guard guard(some_mutex);
    foo();
  }

[kluge]

Ah, now I see it and it makes sense.

The answer to the code review question is obvious. I didn't see the error even though I was told what happens.

[shmerl]

I guess you meant std::lock_guard.

Anyway, the first example creates a temporary object which doesn't lock anything since it will be destroyed right away because it's an rvalue (not sure why would anyone do that for locking, unless not knowingly). The second example creates a proper guard which lives until the end of the scope.