I sometimes temporarily modify the program by for example misspelling a file name of a data base column name just to provoke an error that will execute the error handling code. Or changing a condition from == to != to cause an exception. Not ideal, but at least the new code is executed at least once, so I see that it executes as expected.
Creating tests for some of these cases would involve mocking things out, and then you have to make assumptions that may not hold anyway. That's why I like to see it execute in the real environment at least once.
>I sometimes temporarily modify the program by for example misspelling a file name of a data base column name just to provoke an error that will execute the error handling code.
Yes, a lot of things like that are possible. I remember in one client-server project I worked on, we unplugged the network cable from one PC (while the program was communicating between client and server), to see if we got the correct error message that we had written for that case. And did other stuff like that.
This was done in a project I handled, for the World Bank. I had a team of 9 motivated though young developers (first project for most of them). Near the end, before we sent the software to the client, a colleague was assigned to test the software - testing by a member external to the team, a slightly senior guy, ex-CITIL (a Citi IT subsidiary which had more mature software processes than we did then), though of course we did test the software ourselves a lot, first. I remember what happened:
We prepared the machines and software for the test. Called him over.
He said: I am going to sit down and destroy your software.
We said: Go ahead, please try.
And he tried to. But could not. He only found a few minor cosmetic defects.
Getting up after a few hours of rigorous testing, he said something like - I am happy with your work.
Creating tests for some of these cases would involve mocking things out, and then you have to make assumptions that may not hold anyway. That's why I like to see it execute in the real environment at least once.