There was one winter day that I almost could have been stuck at the office. On the ride in to work, there was considerable slush and snow and when my rear derailleur prevented me from shifting into my lowest 2 gears (of 8), I thought it was a problem with freezing. Since I was already more than halfway there, I pedaled harder the rest of the way (uphill for a portion).
When I got a chance to look it over closely, I discovered one of the idler wheels was seized. It could not be turned at all, even after warming up and trying to work some oil into it. In the upper gears, the chain would slide over the idler with some resistance.
Since the bike was just a few months old, I called the LBS where I bought the bike and they actually drove to my office and picked up the bike. They disassembled the RD and discovered the bearing was completely dry and it was not a true ball bearing, but just a sleeve. It worked fine when they brought it back to me about 1 hour before the end of the work day so I got home on my own.
Shortly afterward, I shopped and found a better model of derailleur with sealed bearings in the idlers so that won't happen again.
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If something doesn't ache, I could be trying harder.