the author might be talking about "expert embarassment", which is very different than tacit knowledge. The expert knows that there is a hole in his domain know

the author might be talking about "expert embarassment", which is very different than tacit knowledge. The expert knows that there is a hole in his domain knowledge, but is exasperated because he can't say for sure what it is! That's quite a legit unknown unknown. Even an expert needs the Beginner-Mind to deal with such situations (as author hints towards end of TFA)