Sometimes it can be an easy mistake, because you're the filmmaker and you probably know the script back to front and sideways, so you're not really hearing the audio, it's triggering the knowledge of the script in your brain so you believe it to be fine. But of course the first time viewer has no idea what the script is, so they have to get all the information from scratch from the audio.

It's happened to me before, so now I pour over my audio (if I fiddle around with it in such a manner), probably too much actually. I find closing my eyes and replaying it over and over making sure I can hear at least 90% of the syllables clearly and then bingo...fruit pie.