A q for the knowledgeable..

I've heard the term "break-in" bandied about now and again where it concerns new electronics. Is this just something the "high-end" folk like to throw around or is there something to it ?

The reason I ask is because if this wasn't something that the EE took into consideration during the design stage then what guarantee is there that after the "break-in" period the unit wouldn't sound worse instead of better ?

Thanks,
Mike