While swimming you can fall through recently formed obsidian.

When swimming it is possible to fall through the obsidian that just generated. I am clearing out lava co i can build a slime farm and after after the third time dying this way i recorded it it. My Armour and tools didn't burn thank god for netherite. I selected 1.16.3 for affected versions even though the clip is in 1.16.1 it has happened across both versions.