I listen to the Freakonomics Radio podcast from time to time, and back in May they aired an episode called "the three hardest words...," which, purportedly, were I don't know. The premise was that people hate to admit ignorance and so they hardly ever say, "I don't know."
Except that in most corpus studies, the head-and-shoulders most common, number one, top-of-the-heap three-word string in English is I don't know (It's a three-word string, not four, since -n't is an inflectional suffix, not just a contraction as is taught in elementary schools, but that's another issue.) For instance, in the 3-grams list from the Corpus of Contemporary American English. I don't know is by far the most frequent 3-gram with 199,110 instances (second is one of the at 167,785). In business meetings, we find the same results. Consider table 3.10 on p. 59 of this book, or table 5.8 on p. 183 of this paper.
Now, these are not mostly "I don't know (period)." Far more commonly, they're "I don't know if..." "I don't know what..." etc., which can often be used as a signal of disagreement rather than as an admission of ignorance. Nevertheless, the data stands in rather stark contradiction to the freaky claim. It looks pretty silly to be saying people should fess up to their ignorance, while basing the argument on a point on which you're so ignorant that you assert the most common phrase is the least (or at least the hardest).
(If you're interested in other freaky foolishness, see Joseph Heath's recent post on their simplistic view of the UK medical system.)