Why bad user interface design keeps me up at night

At around 3:00 this morning, I was awakened from sleep by a high-pitched intermittent beep.

After a few seconds, I heard it again.

I groggily looked at my phone, but I knew that my phone didn't make that noise.

Eventually I figured out that the beeping sound came from the carbon monoxide detector that we installed years and years ago - so long ago, in fact, that my groggy brain first thought that it was a carbon DIOXIDE detector, so I went to close the outside door.

As we slowly woke up, and as the carbon monoxide detector continued to beep, I noticed that a message had appeared on the detector's digital display. It was in calculator font - the font in which, if you hold the number 71077345 upside down you can see SHELLOIL. But the message that was being displayed was unmistakable, even to my groggy brain. L6 - level 6.

Unfortunately, even when I read the message, it was incomprehensible. What does "level 6" mean? Does it mean that I'm already dead, or does it mean that someone is running a car a mile away?

I had already removed the detector from the wall, so I read the back of the detector for a clue. Despite the fine print, and despite my grogginess, I eventually found what I was looking for - and it turned out that the message wasn't "L6."

It was "Lb" - for "low battery." (Although the device plugs into the wall, it has battery backup power.)

I found another battery, but it was also used, so I ended up pulling out the battery altogether. Yes, this meant that there was a risk that I would die of carbon monoxide poisoning within the next three hours, but at that point that was a risk I was willing to take.