Q.What is a Near miss ?

Here is how IMO defines the near miss

A sequence of events and/or conditions that could have resulted in a loss. 

The potential loss could be 

• human injury
• environmental damage
• negative business impact

Let us look at few examples in each of these categories

Sequence of events that could have resulted in human injury

• Thinner in a water bottle was left at the water cooler. This could result in a situation where a crew member drinks this thinner thinking it to be water. But as this was noticed in time and rectified before any incident, it will go as a Near Miss.
• A crew member working without a safety helmet with moving overhead crane in the engine room. 
• A crew member entered an enclosed space without ventilation. Luckily the enclosed space was not oxygen deficient and so crew came out unharmed but this could have led to an accident.

Sequence of events that could have resulted in negative business impact

• A crew member double checks the manifold valves and finds that one manifold was not blanked. The closed valve was holding the cargo pressure. If the valve was leaking, it could have resulted in pollution.
• The Opening/closing speed of hydraulic valves was found to be too high. This could have resulted in pressure surge in the cargo line while closing the valve which could result in rupture of cargo line or cargo hose.

Sequence of events that could have resulted in negative business impact

• Any navigational situation that required extreme measures from the navigator. For example "using the wheel hard over" or use of engine at open sea to avoid a collision.
• During watch hand over, taking over officer finds that ECDIS was being used in the wrong scale.

There can be thousand of such examples of a near miss situation. Each of these examples would have one common point. That there was an event that did not result in injury or damage but had the potential to do so.

I will try to add good examples in this answer as and when it comes to my mind. Do you know any other good example of near miss?