Oh it gets even weirder than that.

One observer can see two events happen simultaneously while another sees them happen at different times.

And EVEN WORSE than that, thanks to length contraction at relativistic speeds, you could have one observer think that a train is contained entirely within a tunnel, but another observer sees the train sticking out both ends of the tunnel at the same time without ever fitting entirely within it.

and/or: One observer objectively masures that object A is longer than object B, while another observer objectively measures that object B is longer than object A.

The two observers are not just hanging out together, of course. They are moving ridiculously fast relative to one another.

The speed of causality is a hell of a drug.