The good old days
Writing and testing COBOL IMS DB programs was a relatively easy process and I was going to say that our testing was 100% rock solid. Then I remembered...
As a junior programmer, working for a bank I was given the job of debugging an abend in a batch program that happened in the privatisation of a large UK company. This was pretty serious stuff. I had about five managers looking over my shoulder at one point. I actually turned on them and told them to sod off and let me get on with it, which remarkably, they did.
This was something I was fairly familiar with, our junior programmers most common error, an 0C7. This was before abend-aid was released so there I was digging through a huge printout. I hasten to add at this point that I hadn't written the program.
To my absolute delight, and free of managers, I spotted the problem. The author had only defined a total field as S9(9).99. The privatisation was so over subscribed that a billion quid and more flowed into the bank's coffers and the good old 999,999,999 was as big a number of pounds as mere mortals could contemplate. To those non 0C7 chaps, it was a case of "overflow", more properly known as a data exception. Apologies for any syntax errors above, it was a long long time ago.
So yes, even in those day, with no random input from screens, people were fallible. I left the bank after 30 months of experience to go contracting. My best decision ever.
Oh, btw, it was Barclays and Jaguar ;O)