this is how I feel when...

...I validate my model for the first time


Seriously, people who start using OLED and the Alloy model simulation at first are shocked at how terrible flawed and naïve their carefully thought out models truly are. Slowly, users develop a deep humility towards modeling and realize how deeply difficult the task is. OLED offers a range of tools for better defining models, once the simulation reveals the flaw.

I can't stress enough the importance of model validation. This is a quote from USA DoD Recomended Practices Guide on Validation:

The validation process establishes the faithfulness of the model or simulation to the thing being represented, the simuland.  Validation provides a crucial piece of evidence to support model or simulation credibility for a particular application.  The validity of a model or simulation also helps reduce the risk. (...) The introductory section of the reference document, A Practitioner’s Perspective on Simulation Validation, adds further insight into the link between model or simulation validity and credibility.

Validation helps you fail gracefully, in the intimacy of your lab or company. It gives you confidence over the produced artifact and credibility with the stakeholders. Without validation such inescapable human error will inevitably be revealed in prime time, at your client's hands. And you may only hope that such flaw does not cause any losses.