It can happen that the MCU is moved into debugWIRE mode, but does not respond after a power-cycle and break with the 'U' on the dW line. Even after switching everything off and then reconnecting, the MCU neither responds to ISP commands nor to a break on the dW line. Using HV programming, one can unprogramm the DWEN fuse, and then everything works again. And sometimes, one can recover the chip with a different adapter.
I cannot reliably reproduce the condition under which that happens, nor was I able to come up with a reliable recovery procedure different from HV programming.
It can happen that the MCU is moved into debugWIRE mode, but does not respond after a power-cycle and break with the 'U' on the dW line. Even after switching everything off and then reconnecting, the MCU neither responds to ISP commands nor to a break on the dW line. Using HV programming, one can unprogramm the DWEN fuse, and then everything works again. And sometimes, one can recover the chip with a different adapter.
I cannot reliably reproduce the condition under which that happens, nor was I able to come up with a reliable recovery procedure different from HV programming.