My bosses bought a computer store recently.  One of their clients ordered toner for his laser printer.  Since none was in stock, my boss tried to call the supplier.  As he was discussing with the supplier, the person at the other end of the line just exclaimed, "Hey! I'm the one who ordered that."  The face of my boss turning red in embarassment, he realised that he called the client instead of supplier.

Priceless moment!