[lisper]

Yes, that's right. Extremely rare. The only reason that the theory works at all is that a single spontaneous collapse can propagate throughout a complex system of entanglements and collapse the whole system.

The nice thing about spontaneous collapse is that it makes a testable prediction: there should be a scale at which the behavior of an isolated system starts to show divergence from quantum predictions. So far that prediction has failed to be demonstrated, but people are still working on it.