One test is never enough!

One part of creating coded tests for software is being able to know when something doesn't work as it should, not just confirming that something does work as expected. 

Imagine if you didn't. Consider a test that tries to verify that given X input you get Y output.

If you write a single test for this, you might be confident that when you input X, you do get Y out.

But, what if you input Z and still get a Y out when you shouldn't?

You probably also need a test for Z input too.

Actually, even having two test cases probably isn't enough.

If you're testing more than a simple boolean input, there are a lot more cases that you should probably be testing too.

I thought this went without saying.

I guess not. 

So now, I've said it.