A common mistake when writing tests is to write test scenarios that you expect will work. That's missing the point. While it's wonderful that a particular parsing routine did what it should do when it's handed perfectly formatted data, that's not very helpful in a real-life scenario.
While you can get perfect data, it's equally likely that you'll get completely corrupted or even garbage data in your code. The goal is to ensure that no matter what horrible things you do to the input data, it will never have a negative effect on the rest of the system.
All input should be validated and sanity checked. If something doesn't seem right, it should be rejected rather than you allowing it to cause issues elsewhere in the code later on.
