The car's windscreen wiper linkage.
In classic fashion, as I drove home on a wet new year's day evening, the wipers on my car (MK1 Focus) stopped working. I could hear the motor operate and the linkage moving, but the wipers themselves didn't seem to be connected to anything.
I limped home, and started on the work of stripping the plastic cowlings off to get at the wipers. Unfortunately Ford decided to use aluminium wiper arms and steel wiper spindles, so despite a lot of swearing, a hammer and every other tool I could think of, the wipers were stuck fast to the spindles.
The next day, I gave my dad a call and he lent me this:
When even that failed to move them at all (indeed, it chipped one of the hardened machine taps trying!) I went out and bought this...
As I'd been advised that heat should help, as the aluminium would expand more than the steel, and help break the corrosion bond.
Within a few minutes I finally had the arms off, and could see the problem. The balljoint between the rod from the motor and the wiper linkage itself had worn so much that it had simply fallen apart. Apparently you can buy repair clips or plates that stop this happening, but unfortunately neither was available at short notice on a saturday morning.
Luckily, when buying my blowtorch I'd picked up a box of assorted nuts, bolts and washers. I then got the power drill and drilled through the balljoint from the "stem" of the ball, down through the ball and then through the cup, and fixed it all together with a 5mm bolt. What this did was let the balljoint take the main forwards/backwards loading, while the bolt stopped it from jumping apart.
Once that was done it was a case of sticking all the plastics back and taking the old girl for her MOT on the tuesday, which she passed with minimal work!