I was playing a survival world placing down tracks for my mine, later on I jumped into my mine-cart to head back up.
I was on top of some obsidian when I decided to get off, and instead of me landed behind the cart I was pushed into lava, only I took no damage and I could not mine blocks. I soon began to fall through the world going far past bedrock level. I was -4000 blocks under bedrock until I decided to get off and get back on.
When I got back on the world I spawned at the end of my track system. Nothing was lost, just felt the need to alert everyone about this bug.

I was playing in 1.16.3 with OpitFine (site won't allow me to put 1.16.3 as affected version) but I'm not sure if that had any part in this glitch.