My source is a middle-aged Korean-American woman who first heard this proverb as a child from her mom. The saying goes, “믿는 도끼에 발등 찍힌다” which translates to “one can hurt one’s foot even from one’s trusted axe”. Her description of what this meant was to not be careless despite being familiar with something.
As a Korean-American, I believe that this proverb provides excellent insight into the Korean people. They tend to be a very cautionary and meticulous people as a whole which even warranted the name the Hermit Kingdom at one point due to its paranoia with others. While, now Korea is a much more modernized nation, this ancient proverb was likely very characteristic of the nation.