A mate has had 3 of these fail in as many weeks on his Mazda 6. I examined the faulty bulbs this afternoon, and discovered that the filament legs are not correctly spot welded to the output terminals, and spring loose with the heat from the bulb.

Probably a batch problem, but anyone with H1 halogen headlight bulbs may want to avoid these for the time being.

In effect the filaments hadn't burnt out, but simply parted company from the blade terminals.

Sounds like a poor quality control issue.

I had this on a 100W bulb, where the filament suddenly came away from the legs at one end.

The bulb had minimal time in the car and on, but I guess after a bit more time on it had decided to undo itself. The filament was in prefect condition, just detached at one end.

I've now got alternatives in, but I think it's older batches of these as the newer ones seem fine.