I suspect that the "worked perfectly the first time" basically implies (whether that is accurate or not) "they only had to create one mask before their tests passed", not that it was entirely bug free. Possibly that the mask faithfully replicated the design, but that the revision of the design they first manufactured still had bugs.

In any case, the main thing is that a lot of MOS' early success came from saving a tremendous amount of money being able to get to market quickly and cheaply compared to the competition thanks to actually having guys like Mensch and Peddle coupled with superior process (though that did not last all that long).