> **Puzzle 4.** List some interesting and important examples of posets that haven't already been listed in other comments in this thread.

git commits appear to form a poset. They don't acquire any of the other properties of ordered sets until after you've resolved all conflicts _and_ are looking at a particular branch on a particular remote. The lack of a total order is one of the biggest complaints newcomers have about git, and the commit log looking at one particular branch is an observation on the commit history (and it is plenty lossy).