> I think such omissions are hard to predict and you won't know where is
> the omission until you encounter it. So if you don't know, how do you
> know what to put in the tests?

True.

> And if it is encountered and fixed, such errors should not happen again.

False.

Such errors *will* happen again -- they are called regressions.  In order to avoid those we add tests, and those tests serve several purposes:

- confirm the problem has been fixed (not every bug is a core-dump)
- confirm that changes in the future don't reintroduce the bug
- communicate past difficulties and possible gotchas to new developers

In short:  add tests; patches are not complete without them.