A few months ago someone reported a bug on an web api that I built. The first thing that I ask was the data that they were trying to send. I took one of the integration test, copy it in another file, put the data the I was given, executed the test and the thing did not failed. I was confident that the bug wasn't in there, so I took a look on the source code of the website where the error occurs (I'm technically still part of their team), turns out the bug was in there.
And that was the first time that tests help me hunt down a bug at work.
Note: Even though I trusted the test, I was still considering that it could be something wrong with it.
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