Is there a mistake that can't be reversed?

Abortion.

Every action good or bad has both intended and unintended consequences. Like the rings in a pool of water when you drop a pebble. We can only sometimes live under the illusion that we undid an error by refusing to see the bigger picture.

Brain transplant! Bad brain into a healthy body! 😈

Time only flows in one direction

One that kills someone.

Murder I guess.