Lava. What will usually happen is, I'll come across a lava flow while exploring a cave and make a mental note that it's there and to be VERY CAREFUL around it. Then I'll come back later for some reason, like to mine some ore in the vicinity or to try and harvest the lava itself, and despite being VERY CAREFUL, I will inevitably misstep and fall in anyway.
One time I found a lava flow and an underground waterfall practically right next to each other, separated by a small area of stone floor. I was being very careful around both, because I didn't want the water to flow into the lava the wrong way and ruin the obsidian farming I was planning to do. I started to pick my way around the edge of the lava pool to get at some Lapis ore on the far side, fell in, climbed out, and somehow managed to panic sufficiently that I couldn't quite make it to the waterfall, which was only a few steps away, in time to dunk myself in the water and prevent my embarrassing, fiery death.
One time I found a lava flow and an underground waterfall practically right next to each other, separated by a small area of stone floor. I was being very careful around both, because I didn't want the water to flow into the lava the wrong way and ruin the obsidian farming I was planning to do. I started to pick my way around the edge of the lava pool to get at some Lapis ore on the far side, fell in, climbed out, and somehow managed to panic sufficiently that I couldn't quite make it to the waterfall, which was only a few steps away, in time to dunk myself in the water and prevent my embarrassing, fiery death.