Are there any impossible events at all? Or are all "impossible" events likely, only they have a very, very small probability?

And in general, is it possible to refute something with 100% probability? For example: is it possible to 100% refute the statement "2+6=3"? Or is there still an infinitesimal chance that this might be a true statement? Answers should preferably include arguments, not just guesses. thank you in advance.