Love this thread idea (and this response). Going to give some of my own (I have tons, hard to remember them all).
-Ankle weights. Running with them, walking around with them, martial arts with them, etc. Yes, I watched a lot of DBZ. And yes, they pretty much destroyed my joints. Thank god I was only 15-16 and could bounce back.
-No lower body training for YEARS. I'm sure lots of us were guilty of this. After wrecking my knees with the ankle weights, I had "bad knees", so surely I couldn't lift any weights. I just ran for lower body development...yeah...
-Running programs out of Muscle and Fitness. I'm sure lots of us are guilty of this one too. Hell, just reading that magazine was a poor choice, although I noticed an uptick in quality a few years back.
-Oh geez, here is a doozy: running a linear periodization powerlifting program when I only had a bench and a smith machine. Imagine performing smith machine deadlifts. I did this for one summer, because I decided I had done enough "bodybuilding" and wanted to get strong like a powerlifter. I did the first program I could find, "modified" it for my equipment, and then wondered why it didn't work. I'm lucky I still have a spine.
I'll think about some others. Some of the dumb things I did ended up working out for the best as far as lessons learned or at least built up some GPP/basic strength, so I'm not including them, but I still have my fair share of just absolutely stupid ideas, haha.