The Big Mistake
When the Big Mistake happens, all the parallel Earths swap portions of themselves, carrying along whatever peoples happen to be on that part. For the first few months, the boundaries between these worlds are visible as Quantum Flux. Quantum Flux is a swirling, opaque, and bright blue wall of cloud which obscures everything on the other side. As the realities stabilize, the Flux fades away.
World Boundaries can cause mutations to those who step through. Prolonged exposure to Quantum Flux can be fatal.