Three guys go to rent a hotel room. The room costs $30, so they each pay $10. Later that night the hotel guy finds out the room is only $25 so he gives $5 to the bell boy to split between the three guys. The bellboy did not know how to split $5 in three ways so he kept $2 for himself and gave $1 to each guy - so the guys only paid $9 apiece right?
If the guys paid $27 and the bellboy kept $2, where is the other dollar?
The first years of my career were spent at a local c.p.a. firm. The training I received there has not only stuck, career-wise, but has carried over into other areas of life. Simply put, the basic tenet is this: always check your work, preferably from a different perspective than that which you used to do it in the first place.
Looking at things from a different viewpoint isn't always easy. It requires changing how we think about things, setting aside preconceived notions. We may need to shut up and really listen to someone on the other side of an argument, trying to understand not only what they believe, but why they believe it. We may need to delve more deeply into a process, to understand how things work in order to understand how to check the results.
While it isn't necessary that our out perspective on things change, learning to see more side of an issue, more ways of looking at things, adds to our ability to understand our own position or the processes around us.
The solution to the riddle? In the comments.