Well, actually it was the crystal, clear dawn of a new day. My bike was loaded, it's lights were flashing, it was poised for the start of another routine commute to work.
I pushed the bike from the garage, straddled it and placed a foot on the pedal to push off. Much to my surprise, it would not budge. Try as I may, the pedals would not go round. No turning of pedals = no turning of wheels = no forward momentum = no riding this bike to work.
I looked down to ascertain the cause of said pedalessness. The chain hung loose between the chain ring and rear derailleur. The chain was knotted and jammed. The light was too poor and the time too short to properly diagnose the problem. Not willing to fore go commuting on such a beautiful day, I swapped lights, bags, pedals and butt to a different bike in my harem.
Upon returning home that evening, I mounted the ailing bike in the repair stand, opened a cold beer, sat upon a nearby stool and studied the offending derailleur...
...notice that the chain was position such that it sat atop the small metal tab that spanned across the two rails of the derailleur cage. This tab is specifically designed to keep the chain from coming out of the cage and doing what it was doing. There is physically no way for the chain to get outside of this tab. I shuddered to think of the evil forces at work to conceive such a sinister act.
The only way to get the chain back in place was to either disassemble the derailleur or open a link in the chain. I chose the latter path and while I was at it decided to disassemble and clean the entire drive train.
As I was cleaning the derailleur on my work bench I noticed two small grooves on top of the aforementioned tab...
Those grooves were the exact width of the chain and bespoke of the chain riding upon this surface for some period of time. Owing solely to my superior powers of observation and deduction, the pieces of this troubling puzzle fell into place and this perplexing mystery was solved. The chain didn't happen to pop outside the derailleur. It didn't make it's way past the small metal tab. The chain traversed this improper position because I mounted it that way. A couple weeks earlier, I had taken the chain off to clean it and when I put it back on, I obviously threaded it incorrectly. So that was why I'd been hearing a clicking sound from the direction of my derailleur.
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