I was on-call to deal with escalated (business) customers, so I had to go in to the office (this is in the days before good home internet) around 10:30 at night on Christmas eve and my wife drove us to the office because I was still talking to the customer on my cell phone.

After a couple hours troubleshooting in the office (still talking to him on my cell phone), I reached over and hit the 'Mute' button on my desk phone and turned to my wife and said something like: "I don't get guys like this, it's 12:30 on Christmas eve and he wants to troubleshoot this stupid problem. Go home, dude!" and then from my cell phone I hear the guy say: "Um... I can hear you..."

I apologized profusely and even sent the guy some swag later, but it was not my smoothest moment.