This question comes directly from an episode of the call-in automotive advice show Car Talk. The original call occurred in or around 1994, but recently re-aired in episode 2349, "We Specialize in All Makes Foreign and Domestic".
In his call, "Fred" (an alias) from southern Georgia tells the following story:
I was playing -- I'm a musician, I play on the weekends down here in Georgia -- and I was playing at a bar. We took a break, went out to the parking lot, and when we were coming back in it was starting to rain. And I noticed that, it was probably about a 1985-86 Ford F-150, had its truck door open. So I didn't want the person's truck to get wet. [...] So I slammed the truck door shut, shut the truck door for him. And the only thing that went through my mind at the time was, "Gee, I hope he has his keys." However, when we were playing about a half hour later, somebody ran into the bar and said, "Good heavens, a truck is on fire." And it just so happened to be the same truck.
[...]
I heard through the grapevine about two weeks later that it was an electrical short circuit, somehow, in the dash or the truck door or whatever.
The hosts' opinion was that, although "Fred" was trying to be a good samaritan and prevent the truck from getting wet, it's entirely plausible for a vehicle with a latent electrical problem to be "pushed over the edge" into failure by the action of closing the door. They conclude that, sad as it may be, the best course of action for anybody who sees a similar situation would be to just leave it alone.
From a legal standpoint, what is "Fred's" true liability here, and how likely is it that he would be found financially responsible for replacing the truck?