Observations and theory both point out that the magnetic photosphere is not smooth but irregular at optically-thin scales. The relevance of these irregularities for a proper magnetic field diagnostics depends upon the not yet known structure of the magnetic field fluctuations (and, of course, on the purpose of our determination). Sánchez Almeida & Lites (1999) analyze quiet Sun network and internetwork Stokes profiles allowing for optically thin fluctuations (i.e., they use MIcro-Structured Magnetic Atmospheres). The retrieved semi-empirical quiet Sun model atmospheres present several conspicuous counter-intuitive properties. If these models contain the essentials of the real Sun then standard quiet Sun magnetic field determinations are seriously biased. This contribution presents and discusses some of these unexpected properties: the IR Fe i 15648 Å line is rather insensitive to strong kG fields; large magnetic field strengths can easily escape detection; unresolved mixed polarities seems to be the rule when dealing with most of the solar surface, that is to say, when measuring the signals that may be carrying most of the solar magnetic flux, etc. The conclusion to be drawn from these results is that accounting for optically-thin fluctuations does matter for a proper quiet Sun magnetic field diagnostics.