A thought experiment is considered on the observation of the instantaneous collapse of an extended
wave packet. According to the relativity of simultaneity, such a collapse being instantaneous in
some reference frame must be a lasting process in other frames. But according to quantum mechanics,
collapse is instantaneous in any frame. The probabilistic nature of quantum-mechanical phenomena
eliminates any contradictions between these two apparently conflicting statements. It is shown that
the consistency of quantum collapse with relativity follows directly from the Born postulate without
any use of mathematical properties of quantum operators, which allows us to discuss this topic in an
introductory course of modern physics. The consistency of quantum mechanics with relativity is also
shown for instant disentanglement of an extended composite system.