I had one of /those/ things happen today. On my way back to the lab from
lunch, having rattled down through the gears for a Give Way line at my
favourite junction I found that I was unable to get more than half way
back up and was left spinning furiously to keep up with the traffic as
I sailed across the line. I struggled the remaining couple of hundred
yards in too low a gear, and had to crawl across the traffic on the right
turn into the back yard.
Of course I checked the usual things in the usual order: the cable was
slack, but it wasn't stuck under the bottom bracket, and it could be
moved freely though the bit of outer by the rear mech. Eventually I
tried brute force, and indeed the mechanism couldn't be presuaded to go
any further out than third. It wasn't bent, and the end-stop wasn't
out of position. I scratched my head (metaphorically) and got a bit
grubbier (literally) poking about here and there. OK, so it's not the
cleanest bike, and it has been commuting in bad weather through recent
wet weather...
Eventually, I caught the glint of someting shiny. It turned out to be
a bit of fresh tar inside the parallelogram, and it was on the corner
of a granite chip in full granite camouflage inside the granite-coloured
dust on the mech. It was sitting in the jaws of the parallelogram and
was just wide enough to hold me in third, without falling out when I
changed down to second or first, but preventing the mech from moving the
other way.
The thing that had been mysterious about this was that the area of
surface dressing where I had kicked up the chippings had been a quarter
of a mile away from where the problem appeared, but I had done that in
top (well, top on the middle chain ring).
So now we know why the loose chippings signs have recommended maximum
speed limits on them.
And then there was the business of getting it out, which involved
escalating through a series of single-part poking-out devices all the
way up to snipe-nosed pliers before someone got me some blu-tack on a
stick. Every toolkit should have one.