I don't think you're alone in what you did.

I was working on an old S plus slant top power supply for the bill validator. I went to push the Molex plug back in to the other side. There was a little resistance but I just pressed it together thinking it's old and just needed a little harder push.
I flipped the power on, but walked across the street to talk to my neighbors when he asked me for something.
Five minutes later, I return to my garage to THAT smell: burning electronic something!

I had put the plug off by a quarter turn! The "resistance" was the NOTCH that I should have lined up.