One of my early jobs in the late 60s/early 70s was with a small government group providing online data processing services to various local hospitals. The system ran on two CDC computers, a 1700[1] and a System 17. These were originally intended for things like process control, but this group had added a home-grown multiuser timeshare system on top, then an online pathology application system on top of that!
My first experience in this group was watching them debug a problem. They had an old FORTRAN listing spread out over a table-sized (10Mb!) disk drive unit. The bug was in 5 lines of code, but it just didn't make any sense - it was as if one line was simply not being executed.
Eventually one of the guys went over to the command console and looked at the object code on disk. "Hey, that 'IF' statement isn't in the object code!". More head scratching ensued, until someone looked more carefully at the grubby thumbprint at the start of that line on the dirty old listing.
Under that grubby thumbprint was ...
[can anyone quess?]