Forty years ago today (July 20, 1969), man first landed on the moon. Mankind also read a computer error message that had never been farther from the Earth.

Before the lunar module began its decent to the moon, astronaut Buzz Aldrin left the docking radar on. He did this as part of his counter-checklist; should an abort be necessary, the docking radar would make it easier to re-dock with the lunar-orbiting command module.

On the way down, mission commander Neil Armstrong noted that they were a little early in their decent. That would mean that they’d miss the intended landing zone.

Then it happened.

Error code 1202 flashed on the computer screen. The astronauts were not trained to deal with that particular error code; the programmers who designed the Apollo’s computer never assumed that it would come up. (Sound familiar?)

Error code 1202 was followed by error code 1201. They were caused by Aldrin leaving the docking radar on. The condition the error codes represent is called a overflow.

Overflow is a common computer problem, especially in the early days when computer memory — specifically bit width — was a big issue. Or rather, it was a tiny issue because the computer could store only so much information.

You’ve probably seen an overflow yourself — and been quite excited about it. Back before cars went digital, they had analog odometers. I remember watching the odometer on my dad’s car roll over from 99,999 to 100,000 miles. But if that odometer had only 5 digits, then when it rolled over from 99,999 it would go back to 00,000. That’s an overflow.

You might have also seen an overflow on a calculator. When the number gets too larger for the calculator’s processor, an E is displayed. That’s an overflow error.

In a computer, an overflow is when you try to stuff one extra bit into a storage place that can no longer hold another bit. A special register in the processor, the flags register, marks the condition as an overflow. It can be interpreted as an error, or it can be used to have the program do something to fix the problem.

On the Apollo’s computer, the overflow signaled an error. Fortunately, it didn’t mean the end of mission. Armstrong was able to manually land on the moon and take that small step. The computer still worked, thanks to the computer scientists at NASA who said everything was okay and, well, they would make sure that errors 1201 and 1202 wouldn’t happen again. And they never did.