Depends on the mistake and the life, Lamb Chop. [ ha ha you have a new nickname and b/c it annoys her, that is 100% the reason why the Doctor keeps using it. ]

Anyone who says that time's linear is wrong. It's a big, messy ball of stuff. Some bits can be replaced, some bits can be extended, and some bits are such a fundamental part of the ball that you can't really do anything with them.