TLDR: almost triggered an automatic fire suppression system at work with a microwave, which could have cost tens of thousands of dollars - and my job.

The dumbest and (almost a) very expensive mistake I made was using a microwave - if that counts as cooking.

I used to work night shifts - alone - in a small computer data center. We had a break room adjacent to the computer room c/w a microwave. Management was sensitive about having cooking appliances in or near the computer room because it was equipped with an automatic Halon fire suppression system with a huge tank of Halon gas and nozzles in the ceiling that would fill the room with fire snuffing gas in seconds. For example, they didn't allow us to have a toaster because that might generate smoke that could accidentally trigger a Halon dump. If the system was triggered and the Halon dumped, it would knock the data center off line for hours, completely disrupt company operations for half a day, automatically have firetrucks rolling, and cost $15,000 to recharge the Halon. So, they allowed us a microwave, because...what harm could a microwave do, right?

One night I brought a frozen Swanson chicken pot pie for my mid-shift meal. I had just put it in the microwave and was punching in the five minute time when the phone rang back at my workstation. I quickly banged in the digits and ran to get the phone.

I got immersed in the problem on the phone and forgot about my turkey pot pie. About 25 minutes later, I (sniff, sniff) noticed a faint burning smell. I looked back to the break room door and was horrified to see that the ceiling of the break room was obscured by two feet of thick smoke. It had filled the break room ceiling and was beginning to spill through the top of the door into the main computer room - which was festooned with smoke detectors in the floor and ceiling.

I raced to the break room and saw dense, lavender-colored smoke ROLLING out of the microwave, which still showed 30 minutes cook time remaining. In my haste to punch in five minutes, I had punched in 55 minutes. I stopped the microwave and closed the door between the computer room and the break room to contain the smoke in the break room that was not equipped with smoke sensors. I did my best to fan smoke away from the detectors in the computer room.

I managed to prevent a dump, and tried to disperse the smoke out another door into the office area of the department, but it stunk up the whole floor and when day people started coming in the morning, everyone was asking about the smell. I could still smell it when I came in the next night.