The importance of executing RETI:
(sorry for answering a question asked to Peter)

If the uC is running an ISR when it resumes to the SW reset, interrupts with the same or lower priority will remain disabled. There is an interrupt controller in the uC that deals with this "internal tasks". The programmer has not direct access to this internal controller, so the trick is to execute the RETI instruction as much times as interrupt levels exist. Remember that RETI is used to return from an ISR, so it restores the earlier state of the interrupt controller.

Alfredo del Rio.