The human factor is a variable that must be included as a variable in everything we do. If your process can be thwarted by someone having a bad day, you haven’t appropriately considered the human factor. If your process assumes always being run by someone perfect and rational, you haven’t appropriately considered the human factor.
We Humans are a fickle bunch. We are often stubborn, obstinate, and rebellious. When things are going fine, we think we can do better. When things aren’t working to our expectations, we think we can improve things on the fly.
People are a variable that is difficult to control but important to account for. Make sure you do.