HV Rescue Shield 2.0 customer Matthew Beckler has uncovered an issue that affects all variants of the Rescue Shield.
Long story short, if the target AVR is configured for a fast internal clock and zero startup time (eg. LFUSE=0x62 on the ATtiny13A gives a 9.6MHz clock with 14CK +0ms startup delay), then a slow rising edge on the 12V RESET signal from the Shield results in the target not entering or dropping out of HVSP or HVPP mode. The symptom is that regardless of the fuses stored on the target, the Rescue Shield fuse reads return 0xFF. In addition, the fuses can’t be changed.
I am currently investigating a solution. There is more information about the issue and updates regarding debugging in the HV Rescue Shield support forum.