I'm such a good mechanic. I failed to tighten the rear axle nuts enough on my bike after swapping in the Motobecane mags, and I took it on a ride about 4 miles and back. On the way back it started making a grinding noise when it was coasting, and I said, this feels really weird, and since I was 3 blocks from home, I just kept rolling. I get it on my lawn and go to pedal to pull it in and it won't pedal. I see there is slack in the pedal chain and then I realize that my wheel is being pushed by the lower edge of the swingarm. This is exactly how it was when I arrived home after that ride!
I took another bike and retraced my steps, but I could only find one tensioner and one nut. :( I need a new moby axle nut.
I would say that was a close one, and also one of the more dumb things I've done recently.