As a postgrad in the late 80s I made some extra book money acting as a helper in the computing lab. A few of us would be posted there, for undergrads to come to for help. This tended to be focussed at the start of the year, when there were groups discovering Unix and programming for the first time.

One time an Irish girl called me over, saying that she couldn’t understand what was going on: she thought her program looked right, but for some reason each time she ran it she got partial output, and varying amounts of output each time. I don’t remember the specifics, but their assignment involved writing a program that was generating various values and writing the results in ascii tabular form to a file.

I had a look at her source code. Everything looked fine. She showed me the file generated by her last run, and indeed it looked truncated. Hmmm. “Ok, can you run your program for me, so I can see what happens?”.

She typed ./a.out and hit return. Her left hand darted to the keyboard and she hit Control-C. I was still mentally processing this when she cat‘d the output, and turned to me: “See!”. It did indeed contain partial output. Again.

“Um, can you just run it again please?”. I figured I must have not seen right. But once again she typed ./a.out, hit return, then whap! she hit Control-C. I asked her why she hit Control-C every time she ran her program: “I discovered that Control-C makes the % prompt come up quicker”.

In her mental model, the % sign appearing meant “the previous command has finished”. The undergrads were all working on a VAX-11/780, which at times was grindingly slow. She’d stumbled across a useful technique for making the prompt pop up more quickly. It hadn’t occurred to her that hitting Control-C was the cause of her problem, she just thought it was making the faulty program run faster.

This has stayed with me ever since, partly because it’s a funny story, but more as a reminder that when you don’t fully explain a system to people, they build up their own mental model about what’s going on, and as long as it fits their observations and experiences, it must be right!