Actually, this is something I discuss right from the very first consultation because I have learned from this mistake!
My very first client was in the Public Relations industry. Some of the tasks she wanted me to do ended up having to be done multiple times because things that were a given to her, were not to me. She obviously had experience in the industry whereas I didn't. The project was a flat rate so I ended up spending WAY more hours working on it than I had anticipated. Luckily she wasn't upset, but it was a bit frustrating.
From that experience forward, whenever I consult with a new (potential) client, I express the importance of communication, explain that it can often be a little rocky in the beginning until we learn each other's work styles, and suggest direct phone contact in the beginning until we gain a solid understanding of each other's needs and wants. As stated, e-mail communication is easily misunderstood. This has worked out very well for me so far. If I have a question, I don't hesitate to ask. I'd rather do it right the first time. I feel confident (now-finally) with my services as a VA and have enough experience to know how to get the most efficiency out of my time. I don't think it displays incompetence in my work as a VA, but rather ignorance in the field of my client. With thorough communication, time and experience, we will overcome that hump and progress into a successful team! That is my goal anyway.
In my opinion, honesty is the best policy!