When I was an undergraduate, we used a locally developed timesharing
system called the Kent Online System (KOS). I was a geek, and was trying
out all the programs I could find. One of them seemed fascinating, but all
I could make it do was spit out error messages.

Eventually I asked a professor, and it turned out that he'd written it as
part of his Ph.D. He explained that it was a general purpose macro
processor called ML/I, and showered me with documentation. It transpired
that this included instructions for using it to translate itself for a new
system. The year after, I did just that on a minicomputer to which I had
access in the Electronics department (it was a Honeywell DDP-516, which
some may know from ARPAnet).

Fast forward another year, and I'm doing a Master's at the University of
Essex. My speciality was (and still is, as many people know) operating
systems, and I got to know a Ph.D. student researching that area. We
chatted, and I found out that he was manually translating that macro
processor for a 12 bit minicomputer (it was such a tight fit that it had
to be done manually). This was a spare time project for him. For those who
are interested, it was a PDP-8.

A few days later, I met him on the stairs. I asked him how the macro
processor was doing, and he said it was all fine except for one weird bug.
When it produced any error message, it forgot something called the 'insert
environment'. Perhaps I could look at the code sometime?

I replied that I didn't need to. He was corrupting the fourth word from
the bottom of the stack, probably due to a faulty backward move routine
with an off by one error. I then rushed off to my lecture.

Next day, I saw him again. He was most impressed, because I was exactly
right, and I hadn't even seen the code.

I never told him that the day before he told me about the bug, I'd been
reading the documentation on the data structures used by ML/I ...

That prof eventually handed over maintenance of ML/I to me. It is still in
use, and I run a website for it. The prof sadly passed away a while ago.

--
Using UNIX since v6 (1975)...

Use the BIG mirror service in the UK:
http://www.mirrorservice.org