‘The error within a procedure, or steps, that causes the results not to be as intended. By way of contrast human error does not involve the procedure itself, but instead it arises from a mistake that the person performing the procedure makes which leads to undesired or unexpected results. This can easily be caused by any minor detail being omitted or carried out incorrectly.
Reference: Adapted from What is the difference between a procedural error and a human error in an experiment. http://www.science-mathematics.com/Chemistry/201111/18744.htm