I am reading the book "Space and Time in Special Relativity" by David Mermin. In chapter 13, at page 128 in my print, he says the following (screenshot):
I'm referring specifically to the sentence "Note that this is the principle of the invariance of coincidence again". I had to reread it to see that he meant spacetime coincidence, not just time coincidence. Mathematically, that makes sense; a single point with a single spacetime coördinate will always have just 1 coördinate, no matter what coördinate system is used.
But why is this a principle at all? How can two events occupy the same spacetime coördinate?
I noticed someone asked a similar question here: Coincidence of spacetime events & Lorentz invariance
And one user answered that in this case, coincidence means identical. However, it seems to be used as an argument to say that the principle has no meaning in reality.
Are there any examples of differrent events that occur in the same spacetime coördinate?