What not to test when it comes to Unit Testing?

Question

In which parts of a project writing unit tests is nearly or really impossible? Data access? ftp?

If there is an answer to this question then %100 coverage is a myth, isn't it?

Answer 1

Here I found (via haacked something Michael Feathers says that can be an answer:

He says,

A test is not a unit test if:

• It talks to the database
• It communicates across the network
• It touches the file system
• It can't run at the same time as any of your other unit tests
• You have to do special things to your environment (such as editing config files) to run it.

Again in same article he adds:

Generally, unit tests are supposed to be small, they test a method or the interaction of a couple of methods. When you pull the database, sockets, or file system access into your unit tests, they are not really about those methods any more; they are about the integration of your code with that other software.