What are some common mistakes in audiobook production, and how can they be avoided?

The most common mistake in audiobook production is speaking too fast. This is an audiobook, not a radio announcement. The second most common is going through and cutting out the breaths in post-production. People breathe. The brain expects to hear the narrator breathing and if the breaths are cut, it sounds robotic. Breaths are a part of the performance, the same as fret sounds on a live guitar or breath sounds in classical woodwinds. A trained narrator learns how to breathe in a way that is not intrusive. If your narrator speaks too fast or has not been trained in proper breath technique, they're not the person for the job. Let them practice on someone else's dream.