My car's been tending to stall when I put the headlights on, so I investigated and found the earth terminal on the wiring connector that attaches to one of the bulbs was partly melted.
The earth appears to be OK, it has the same resistance to battery negative as the other side, gets the same voltage, so I suspected the bulb and changed it for a spare.
The bulb's earth terminal is a darker colour than the others, presumably through heat.
Is it heard-of for a bulb to go bad like this? That is, to develop (presumably) an internal resistance or something but still work.