My car started acting bizarrely lately and the engine lamp would light up. Terrified I remembered I hadn't changed sparkplugs for ages and went to the mechanic for that.
There he informed me that the car showed cylinder misfire error and that he changed the spark plugs. He also told me that the spark plug cables were worn (!!!) and would need replacement sooner or later. The "problem" was that the ignition coil should change as well since it was one part with the cables.
A week later, the average consumption had increased and then the dreaded engine light turned on again. This time, I plugged in an OBD2 adapter I had gotten cheaply some time ago, and it showed cylinder misfire.
Reading a bit around I understood that a cylinder misfire might be a hard problem to diagnose. In any case I went in ebay, found the relevant part 80euros cheaper than the mechanic told me it would cost, and left the car in the parking lot for 2 weeks, waiting for the part to arrive :(
The part eventually arrived and the mechanic did the exchange it for 15 euros. I asked him to keep the old one for me to see and he promptly gave it to me.
The cables were in BAD shape. the insulation had worn off in a few places (strange for non moving parts, so I assume the mechanic or someone else, had misused them) and one spark plug connector had insulating tape around it for some reason. The thought that someone had changed it deliberately crossed my mind. But there is no way to prove it of course. The car is like a black box. Unless you open it up and look closely inside, there is no way to know what exactly is in there.
Next time I should have a closer look in there, though the model of the car doesn't help me much with so little space under the hood