The increasing organizational complexity and competitiveness in industry is driving the interest in project management to ensure successful businesses. Uncertainty in estimating activities’ duration is one of the main criticalities of the planning phase, especially in the industrial environment, where most projects, such as the optimization or qualification of new lines, usually require the involvement of resources whose commitment is almost never bound to just one task. In this context, the impact of human behaviour if not properly considered, can lead to serious damage in terms of time and costs and these hidden risks need to be properly assessed. The present study aims to analyse and quantify the impact of human behaviours (as described by Parkinson’s Law, Student Syndrome and hidden safety) on the project in terms of delays in scheduled time though the use of a Monte Carlo simulation. The objective of the methodology is to provide the project manager with the insight required to consciously address the uncertainties of the scheduling phase in order to ensure the success of the project. In order to illustrate the methodology in a more comprehensive way, a case study regarding a project for the innovation of the production process in a real industrial plant is provided.