Regarding cautions: Well, I am not continuing to power it this way pending more knowledge, but thanks for the warnings.
The 1ma limit mentioned for protection diodes is the kind of info I did not have, tho, so I'm glad I asked.
I would disagree somewhat about VCC. VCC input is floating, not grounded at 0V. This is not a case where there's a power supply trying hard to keep the other end of the diode at a fixed voltage. In fact the floating VCC rail is being raised via that same diode to about one diode drop below the D8 digital input; still apparently within the chip's operating voltage so the rest of the chip seems to be woking.
The current limit (cited above as 1ma) for the protection diode is a larger concern, especially if off-chip LEDs are being powered.
The main lesson here is what I said last time - it's something for us to be aware of, that a chip could actually operate (and accidentally actuate some output) if some input pin is driven to 5v from a different supply. I did not know that.