Not long after I started I remember sending a letter out to potential customers introducing ourselves, as we had a few big name companies using us and part of the letter was something like "trusted by xxxx to deliver"
My only competitor at the time got hold of one of the letters and passed them on to companies mentioned.
One of them wasn't too happy, and I was summoned in for a meeting, and in the end we lost the work, and our competitor took over.
At the time I was incredibly annoyed, but I'm a great believer in karma.
As a big customer it was 90 days for payment, 5 days after we received the last cheque they went into administration, so he did me a favour as I would never have survived the loss :)