I'm usually pretty methodical and patient with my builds, so haven't done anything truly dumb. This one is from my buddy's younger brother:
Back in late 80's - early 90's, CD-rom drives were pretty expensive (for 2x speed, mind you)... several hundred bucks. My buddy and I were going to his dad's store to install a new drive in one of the computers. His younger brother, who was always trying to one-up us, decided to install the drive before we got there. Well, for some reason he couldn't get the molex power connector to fit into the drive. No matter how much he forced it, it would not go in. He pulled the drive out, took a good look at the power connector and noted the angled parts on the corners. Well, looking at the molex connector in his hand, he sees the side with square edges. Well, when you're a pre-teen, working in a hobby store, everything looks like an excuse to use tools. He decides to fix the plug corners so it'll go into the connector on the drive. Literally grabs a file and cuts 45's on the edges. Plugs in just fine. Gets it done and all buttoned up just as we walk in. He's got this fantastic grin on his face as he flips the power switch. Drive makes the oddest sound I have ever heard come out of a computer, followed by a pop and rather large amounts of smoke. We turned around and walked out, and left him to sort that one out.