General: Burnin' VGA Connector

My little brother came home with a trunk full of old PCs (Pentium 2 or 3). Today, we tested some of them. With the first one everything was fine. Adding a HDD and an old SDram, it booted up in basic config.

When we tried the second one, smoke came out of the AGP port. OK, we replaced the video card but nothing happened, the screen stayed black. Same with another PC with onboard graphics. The next one had a config similar to the first one. it booted up, but suddenly, much smoke came out of the VGA-connector. When we unplugged it, the middle pin of the middle row was still sticking in the graphics card plug, almost glowing and the plastic around totally melted. My brother did a quick check, it seems to be the pin for the shielding of the cable responsible for "blue". As the monitor was running fine and showing the bootscreen until we disconnected it, I don't think that it suffered too much damage. My brother is about to solder a spare connector to the monitor cable, but I told him not to use it again before we have not found out some things:

Where did the power to melt the pin come from?
And how can we ensure that this can't happen again?

Have you any ideas? It was the middle pin of the middle of the three rows, the pin next to it is unused and missing.

Well, we have a piece of cable with a VGA connector. We'll cut off the burnt connector and replace it with that. Actually, there is not enough left from the burnt connector that we could just solder in a new pin. The hole that was burnt into the plastic is much too big for that.

But I simply can't understand why this happened. It was one of the shielding /grounding pins that burned the plastic. I can't imagine where such a high current can occur in a VGA-connection, except in case of a severe short circuit. If that's true and I don't find the source of the short circuit, it will most likley happen again. Quite a dangerous game...

We've turned the responsible machine into spare parts and repaired the monitor cable (we soldered & taped it-looks strange but it works ). The monitor is still working, but there's a little "artifact" left when something is displayed on screen (due to the cut-off shielding of the cable, I assume).
Since then, we were more careful with our test runs. My brother came up with some more PCs (e.g. cool Pentium 2 and 3-based double-processor servers or a six drive CD server) and we managed to bring most of them back to life, even if we don't know what we want to do with all that stuff.

It was just a piece of cable with a completely fixed connector that we had in spare. A monitor with a rundabout 20cm (or 8 inch) VGA cable would be a bit odd , so we cut off the burned one and connected the new connector with the old cable.
At least, the monitor works again.