I have a Mercedes 250D stw from 1995. Works like charm, only 375000 km driven.
Now to the nightmare part: Sometime last spring the infrared locking/start prevention system started having hiccups. There's two receivers, one in the driver's door and another in the trunk and if the former wouldn't work, the latter would. Mostly. And it's not the key, I have two and both either work or don't. Now, one evening after work I was planning to go home. Couldn't get the lock to open! I kept pressing the key for one hour and a half until I called my neighbour who towed me to the local dedicated MB service. Hoping some bump had fixed the issue I tried the key again to no avail, so I put the key in an envelope with a brief note and dropped it through the door mail slot. Next morning the repair chief called and asked if the error would be of the nature of fixing itself... I said yes, but this time it didn't. They spent most of the day searching for the culprit but couldn't find any. Not with their equipment nor by checking every joint and relay concerned. The spare part guys said they have never been asked for the components concerning this issue, and at least one of them was working there already when the car was new! Neither has my regular one man MB unofficial dedicated repairer heard of such. Even the Finnish MB bulletin board has been quiet after the first obvious question.
The components would cost about half of the value of the car and there's no guarantee that they would fix the issue. And since the components in question have to be ordered directly from the factory, being coded for my vehicle only, there'd be no possibility to return or resell them either...