The joke is that these two things seem trivial (perhaps even outside the scope of the real “problem solving” parts of the job) and computing is full of apparent

The joke is that these two things seem trivial (perhaps even outside the scope of the real “problem solving” parts of the job) and computing is full of apparently much trickier problems. But in practice everyone repeatedly runs into trouble with the same couple basic issues. It’s a joke because it’s not literally true – the other problems in computing can be incredibly hard, sometimes taking years of work to figure out – but like any good joke it does also have a kernel of truth and hopefully makes the listener do some reflection. Adding “off by one errors” is a riff which only makes sense once people are already familiar with the original joke. It can be funny but also kind of misses/obscures the original point.