What principle do you believe that MWI is violating that is analogous to a perpetual motion machine violating conservation of energy?

In the case of the perpetual motion machine, it is easy to see that the described system violates energy conservation, because you can compare the energy in the system at different times. From this global violation, one can deduce that there was a mistake somewhere in the calculations that predicted it for a system that follows the physical laws that imply conservation of energy.

So, what is the global problem with MWI that leads you to believe that it has a PoC flaw?