What are your most effective tests?

Hi everybody,
I would like to know what are youre most effective test and how you would categorise them.
By effective, I mean that they catch faults.

My most effectives tests are those that make sure the application does not do what it is not supposed to do.
But requirements, mock-up or whatever is available to describe features are formulated to describe the normally expected use.
So getting around to flesh out test cases that are outside the expected usage is not a simple process.
I do not have an easy way to get there.

So…
What are your most effective tests?
Do you have a routine or a process to flesh out those tests. ❓