A deceptively simple question: What is, was, and will always be impossible to occur?

I'm having difficulty answering this question with any certainty.

At first glance, most people would agree that some things are impossible.
But then have a difficult time giving an example.

The imagination allows us to think of something seemingly "impossible" and then in the next thought, to think of a way that it might be possible.

Is this a true reflection of reality?
Might nothing be impossible?

Impassable numbers such as the speed of light and absolute zero are not as absolute as they were once considered to be.

This type of thinking leads to the suggestion that all endeavors will eventually succeed and that any idea put forth at one point was, or will be valid.

Dare I say, might a god exist, just not yet?
Can any of this be true?

As a caveat, this question does not concern labels.
It is impossible for a triangle to have 4 sides, but it is possible for a triangle to be modified in such a way that it now possess 4 sides. The label of triangle is no longer appropriate, but it was never meant to be binding in the first place.