What's the dumbest, facepalm-inducing thing you've ever done in attempt to fix a bicycle, or just anything else related to cycling? This was prompted by the guy who somehow got an entire bottle of glue in his bike. I'll go first.
So, I would call my try more "ambitious" and "poorly informed" than truly stupid. Was trying to fix GF's creaky BB. I wanted to do the work myself, and get a BB online. Of course, she had the cursed ISIS crank interface, and I only had the standard puller. I tried a technique I'd read about on the interwebs: using coins to make the puller bottom out faster (yeah, yeah always use the proper tool...)
Things were great, until the handle was turning very easily, but the cranks weren't moving. Of course, the puller went straight through the penny. So I thought I'd stack a couple and try again. Same result. In the wee hours of the morning, I start to get careless and destructive. Finally, I'd stuffed a half hour's worth of parking in there and stripped the crank threads (I'd finally decided that even destructive removal was ok, as I wanted to replace the garbage cranks for her anyway). Nothing budged, and I paid extra labor to have the stripped cranks removed at my LBS.
Not that bad, just ended up costing me more than it should have.