Something happened in which somebody seemed to make an error.

The important thing here is to look really closely at what type of error it really is, and where it comes from. They can be subdivided into several different types of errors:

• Slip (oops): Slips are often not so disastrous because they are noticed quickly and usually do not lead to damage or injury. However, every system that by design encourages slips is unacceptable

• Lapse (omissions - something not done): These are more dangerous because they often go unnoticed (example: forgetting to replace an o-ring). The problem can get worse because people often think they did do it, so extra checks are necessary. They are especially dangerous during maintenance, where the problem can stay unnoticed for some time.

• Mistake: Mistakes are more dangerous than omissions because people that make a mistake think they are doing the right thing. They can be so sure of themselves that they will not see the facts that say otherwise. Example: Somebody is behind in his operational training, whilst the system has already been changed. His trained reaction, based on the old system parameters, can lead to a mistake.