I've been experimenting with HVSP on some ATtinies again, and this time I wondered what happens if 12V is applied to a running AVR. It's obviously 12V tolerant for some 10s of us after startup in order for it to enter HVSP mode.
I used a t13 powered from a USBasp, and then manually connected 12V to the RESET pin. The USBasp immediately disconnected, so I removed the 12V. I removed then re-connected the USBasp, and the t13 looked dead. It still responded to serial programming commands, properly reporting the signature. When I switched the power from 5V to 3V3, the t13 started working again. Repeated tests at 5V always failed, and 3V3 always worked.
I'm not sure why the t13 was damaged. Even though there's no clamping diodes to Vcc, there must be some path from RESET to VCC. I did some separate tests applying only 12V to RESET (VCC floating), and with the t85s I tested, I measured around 1V on VCC. With a couple t13s, I measured 1.6-2V. An equivalent schematic for the RESET pin would help explain things, but I have never been able to find one.