@ploum Mostly we didn't find debugging hard, of course, because by the time we'd run the project what used to be regarded as "properly" there were few to no bugs.

But I do remember one.

The system had to drive a bank of modems. Modems and phone lines cost money. So our internal testing wasn't at the full scale of the production system.

We weren't stupid enough to test with only a single modem, so we used a bank of, I think, four.

And the system then failed when tried out on the production hardware which had, I think, sixteen modems.

Because ...

... somewhere along the line the way that you addressed a modem involved sending its number as a string of ASCII digits, and we only tested addresses 0..3, and when we got to modem no. 10, with a two digit address, it didn't work.