I'm just waking up from a 4 hour night time nap since I was working too much to get any sleep and I thought of an interesting topic. So what was everybodies biggest oopsie/mess up as a technician for a client? Just give the details of the event in all its horror

My personal biggest oopsie I ever did was in my first weeks of training as an apprentice in a computer shop. We had a client who just needed the jammed screws removed from the mounting plate for their printer port. My boss/mentor decides he has to leave for a few hours and the computer was taken apart to see if we could get a better angle on it. He suggests I put the computer together, test the machine and call the client up. Should be easy for anyone right? Well not for me. I start the PC up and the modem won't dial out. I call my boss up and he tells me check the drivers. I check the drivers and everything is fine. So I reseat the modem still nothing. I call him and he won't answer. I try changing the IRQs as he had shown me. Still no luck. He still won't answer his phone. I stress a bit and try anything and everything that comes to mind. Then I notice something. (Dramatic pause) I see that the modem has this interesting looking port on the side of it inside the case. Interesting I say. I wonder if I forgot to plug the modem into something. I do a quick scan inside the case and low and behold I find a plug that is shaped just like the odd configuration on the modem. JACK POT! So I plug the cord into the modem and power the computer on. Now when you watch movies and you hear the sounds of electronics blowing up say for example in Star Wars. Those sounds DO HAPPEN. I also want to say when sparks fly from a computer in a movie that HAPPENS TOO. So here I am watching the most fantastic micro-thermonuclear event in all of the small town of Okmulgee! Mind you this is also burning streaks of light into my retinas that I couldn't get rid of for a full hour. I freak out I don't know what to do! I'm in shock! Suddenly I realize quit watching and do something! I quickly yank the power from the computer and watch the acrid little black stream of smoke come up from what used to be the capacitors on the poor modem. I pace around the store worried, scared and resigned. Not only did I fry a clients computer but I'm sure I'll get yelled at, docked pay, and fired. I quickly go back to the computer in hopes that my lacking skill will miracle it not to be a dead computer. While looking at the inside of the computer I notice the plug that I put into the modem leads directly to the power supply. Being a rookie this doesn't click to me that this shouldn't be a marriage allowed even in the most depraved parts of the world. I do unplug the cord out of knowing by deductive reasoning that I should have things where I started and see if I can salvage it. I turn the computer on and nothing. Amazing really I mean it only worked 10 minutes ago. So I regretfully call my boss and "OH LOOKIE HERE!" He answeres so I freak out and ramble on at a million miles a minute about the whole thing. I even promise to pay for the replacement of the unit. He amazingly is understanding and says to me hey you're learning don't worry about it and to make a very long story short we replaced their unit and I got an interesting story to remind myself of every so often when I need a giggle.