The only reader-added tag as yet is "Cause identified".

I KNEW he'd had too much voltage going in. So it was a faulty regulator rather than a forgotten one.

Great. I assume this means one should separately and rigorously test every component before use... What fun !

I'm remembering the electronics course I took for my CS degree. I'd done breadboard work before, and looking at the first lab I saw a couple diagrams that I could easily put together in a couple minutes. The time allotted for the lab seemed to be more for those who hadn't done any work before... I'd just be in and out. After some really bizarre behaviour putting together the first diagram (including burning out an LED even though I had put a resistor in for it) I figured out the truth. All the stuff we were working with was junk drawer quality (as this wasn't a course for actual hardware engineers). Every piece had to be tested before it was used.