Melted headlamp bulb socket?

[mmh]

Driving home last night I noticed my nearside headlight wasn't working.

I got out to check the connection and found that the top of the 3 prongs on the H4 bulb had melted the socket and snapped off when I tried to pull the socket off.

Anyone know what could have caused this to happen or has this happened to anyone else.

Thanks, Mo

p.s. these are standard 12v/55w ice blue type H4 bulbs