Dangerous common mistakes, what do you think are the most commonly unknown dangerous mistakes that people make without even realizing the impact?

When I keep to ICT related mistakes, the most common mistake is implementing software solutions with an ICT centric point of view, and neglecting the required business functionality.
This can have an enourmous impact on your business, or even society.
  I see errors based on this principle every day.
The most common examples are errors on data validation, like address and entry screen that accepts only validated national postal codes and telephone numbers, and so blocks entry of foreign customers.
If you have a webshop, these errors are expensive.
  But also errors in alignment with business procedures, or ridiculously complex user interfaces are too common.
A too complex user interface in new implemented car registration software in the ministry of economy caused so a lock-up of new car registrations and a considerable economic damage.

I think that the major mistake is classified of expenses by wrong because the right classified result in make decision is right.

Yes.
We all are human being and so we do the mistakes without realizing the consequences in many instances.

Having a baby without thinking it through first.

But what are those mistakes ? thats the question :)