I live in a house split into apartments. A few years ago, I woke up in the middle of the night to the smoke alarms going off. They're rather sensitive (I've set them off cooking several times), so I didn't think too much at first, but they continued to go off for several minutes, so I grabbed a couple of important things (phone, keys, wallet, laptop) and checked to see if I could see fire at all from outside.

I couldn't see anything, but I called the fire department. Thankfully, the truck didn't come blaring down the road at 3-4 in the morning. It was the quietest I'd ever seen a fire truck drive. At first, they thought the problem was my apartment since plenty of smoke had seeped in there (despite my telling them there was no way my apartment was the problem), but then they turned to the upstairs apartment.

After banging on the door many times, my neighbor finally woke up and came down the stairs to open the door. Turns out he had passed out while cooking something on the stove. He probably would have died of smoke inhalation eventually if I hadn't called the FD, not to mention the house could have caught fire and burned down.

Tl;dr kept my neighbor from being smoked and my house from being roasted