A few years ago, I got a call from a client. He was saying he had a problem with one of his HL7 interface (communication standard in the health industry). So basically, he was saying that a part of the communication was not working randomly. But the thing is that the feature he said was in problem was not yet installed at this client (it was still beta and only in 1 place).
So I tell the client that it's impossible that this thing is failing because it's just not installed. But client was convinced there was a problem, so I am polite and continue asking questions. So I ask him if it's totally random or if it happens at a specific hour. Then he tells me it often happens during lunch time.
So, having a good idea of what the problem was, I ask him to go ask if, by any possibility, a clerk would be doing the exact same thing this interface is supposed to do, but manually.
A few hours later, the client call me back, and confirm me that the "interface" was down when it was on lunch break!
