When an event is almost certain to happen, its complement will be an unusual event?

It is true that the complement will be an unusual event when an event is almost certain to happen.   The complement of any event A is the event [not A] in probability theory, i.e. the event that A does not occur.

An event whose probability is small is called an unusual event. The most commonly used value is 0.05, in other words, an unusual event is a probability less that is than this value, and the cutoff value can be any  small value that seems suitable.

Let’s take this for example:

It can either land showing “heads” or “tails” if a typical coin is tossed and one assumes that it cannot land on its edge.