Imagine two cups moving towards each other. Principle of relativity says that we won't know which is moving and which is stationary right?
If either one comes to a stop, we can conclude from the perspective of one cup, that the other cup has decelerated.
But now repeat this experiment with water in the cups.
The one that spills water upon stopping is the one that was moving
This seems to contradict the principle of relativity that it's the same for all reference frames, because only one of the two cups spilled water.