After Christmas this year, I removed the tree from our living room, and in the process of being moved, it shed of needles everywhere. I swept them up, but I missed a few areas on my first pass. So I did a second pass, but when I looked again, I saw there were still a handful left. It struck me how removing >99% of the needles was nowhere near acceptable! Lots of cleaning jobs are like this, I suppose, because even a tiny mess can be visually distinct. In fact, as you approach 100%, the remaining mess stands out more.