Jurgen M said:
Different observers, different results.
Better is: different frames of reference, different measurements.
This has always been the case. We routinely talk about, say, a car having "stopped" at traffic lights. But, the surface of the Earth is rotating at about ##1000 km/h## and the Earth is orbitting the Sun at about ##30,000 m/s##. The car is only at rest relative to the surface of the Earth.
Also, we may consider a collision in the "lab" frame where one object is at rest and another object is moving towards it; and, in the centre of mass/zero momentum frame where the objects are moving with equal and opposite momenta towards each other.
Sometimes it's a problem even to convince students that motion is relative and that both the "lab" and "zero momentum" frames are valid reference frames in which Newton's laws hold.
And, if the student ovrecomes that hurdle and then starts to learn SR, there is then the intellectual hurdle that lengths, elapsed times and simultaneity are also frame-dependent. This is the point at which a lot more students have problems understanding modern physics.
Nevertheless, lengths, times between events and simultaneous events are frame dependent.