This may seem dumb to you more knowledgeable bike mechanics, but I had an interesting mechanical experience yesterday.  I was about halfway through a 62 mile ride, when I noticed a new creaking sound coming from the area of my pedals/cranks/BB.  It only happened when I was pedaling with a pretty good load on the chain, like when climbing a steep hill.
 
After watching my rings go around for a while and trying to figure out what the problem was, I noticed that the chain was drifting right and left as I rotated the cranks.  I pulled over to get a better look, and I found that I could wiggle the rings and crank arms side-to-side slightly with just finger pressure.  When I got home, I found that my 8mm wrench fit perfectly into the bolt in the center of the cranks, and that they were pretty loose.  I tightened them down, and the wobble went right away.
 
It never even ocurred to me that cranks might work themselves loose, but I guess this falls under the old maintenance tip of periodically checking all bolts and nuts on your bike for tightness.  

Holy crap, thanks Bud.  I'll go check mine right now!

Your so right Paul, my headlight vibrated loose while on the Tour Dallas ride. The streets from White Rock Lake aren't the smoothest in my riding experiences that's for sure. I just caught the light in time as it had already slid upside down and had to carefully reposition it and tighten up as I carefully rode toward the last few turns of the ride.