Minecraft wiki entry for Things_not_to_do, associated text: Falling into a pool of lava, which can randomly generate underground, and under Y=10 (1.17 and earlier) all caves are completely filled with lava.
Accidentally breaking into a cave system, underground ravines, or dungeons and take significant fall damage along with possibly getting swarmed by mobs.
The player can also get stuck in the hole they just dug. In that case, placing ladders or pillar jumping should get the player out.