Suppose, in inertial reference frame $F_1$, observers A and B are at rest, each having torch, and are separated by some distance and we have put machine M at middle of A and B.
Machine M has light bulbs on both sides ,right and left, so that if it catches light from A which is at left ,then machine M glows left light bulb, similar with right bulb.Also, if it senses both reaching at same instant of time then it start to make noise.
Now consider another inertial reference frame $F_2$ which is moving at constant speed $v$ with respect to $F_1$ to the right.
Now ,in frame $F_1$ , both A and B turn on their torches at same instant of time, say $t=0$ and both rays reach at M at $t=t_1$, and machine M makes noise indicating that those events were "simultaneous" in $F_1$.
Now we know these events are not simultaneous in $F_2$, in other words ,person seating in $F_2$ will say ,"I should not hear sound from machine M." But somehow machine makes noise.(or it doesn't make noise?)
So does this mean according to $F_2$ ,machine is malfunctioning?