Last night I discovered both of my headlight bulbs' low beam filaments blew out.  Brights were fine.  All other bulbs are fine.  I don't know if it happened right then, or if it happened sometime throughout the day (with the daytime running lights).
 
Anybody got a theory for an electrical gremlin that could be responsible?  The bulbs were less than a year old, both replaced at the same time (so I had a still-good spare bulb in the glovebox, since only one bulb had actually gone bad before), and it's been dry here for days.

I've had both headlights burn out within a day or so of each other before, but never, ever, both at once.  It's freakin' me out.

I have had it happen to me before! Unexplained and it was fine after. Prob a spike or something?

I have had it happen to me before! Unexplained and it was fine after. Prob a spike or something?

I'm leaning toward that explanation, but I still can't wrap my head around two bulbs blowing instead of a fuse.

Come to think of it, is there even a headlight fuse?  Maybe the bulbs are their own fuses?

I've had everything blown, both low beams, then while driving with the high beams on to get home, one blew.... then the other.
I replaced the bulbs and all was fine. One year later, I noticed something green on the passenger side inner fender well wire harness - it was fuzzy corrosion. Not sure if it was related, but I was having a horrible time with my headlights turning on by themselves whenever there was a hard rain. Haven't had a big rain yet to find out if it was the exact problem.

This would be a good time to measure, even monitor, your battery charging voltage. A small, cheap Digital Volt Meter (DVM) will work.  Harbor Freight frequently has them on sale for $3.  The life of tungsten filaments is very sensitive to applied voltage. That means even a 10% increase in applied voltage results in only one-third of the design life (it's a 12th power relationship!). The charging voltage should be between 13.5 and 14.5. It's possible a bad sense wire or regulator is causing the alternator to run uncontrolled.
 
Of course, this may not be it, but it is a good place to start. Headlight bulbs have a well-defined wear-out mechanism and generally fail within a short time of each other. When the first one blows, the second is not far behind -- replace both, as you did. 

This would be a good time to measure, even monitor, your battery charging voltage. A small, cheap Digital Volt Meter (DVM) will work.  

I've got one.  Is there a preferred circuit in the cabin where I should wire it?

I think he means to check the running voltage, right at the battery first, while running, than at the lights themselves, while running.

Ooookay.  An update...

I've found that high-performance bulbs burn out damned quickly on my truck ('02 XL7 for this conversation).  If I get 6-months, I'd be surprised.  I haven't had both go out at once again, but I continue to burn through 'em.  Normal halogen 9003s last pretty long.

I'm currently of the belief that it's been happening because of the automatic-on (light-sensor) feature.

If I turn the headlight switch 'ON', then turn on the brights, the low-beams turn off.  BUT, if the headlights (low-beams) are on due to the light sensor turning them on automatically, and I don't turn 'ON' the headlight switch manually, when I turn on the brights, the low-beams also remain on.

What I think is happening is that I'm cooking the hotter high-performance bulbs by running the brights with the lows remaining on.  I love the extra brightness, but damn, expensive bulbs get expensive!

Anybody else notice this?

I ran into the same issue with the bulb burning out.  Your probably correct in that the driving lights take a lot of the life out of your bulbs, there have been a few write-ups on how to disable the running lights.  I believe the relay is under the dash.

I just ordered a pair of bulbs from Summit Racing for 11 per bulb.  We'll see how they hold up.

Ooookay.  An update...

I've found that high-performance bulbs burn out damned quickly on my truck ('02 XL7 for this conversation).  If I get 6-months, I'd be surprised.  I haven't had both go out at once again, but I continue to burn through 'em.  Normal halogen 9003s last pretty long.

I'm currently of the belief that it's been happening because of the automatic-on (light-sensor) feature.

If I turn the headlight switch 'ON', then turn on the brights, the low-beams turn off.  BUT, if the headlights (low-beams) are on due to the light sensor turning them on automatically, and I don't turn 'ON' the headlight switch manually, when I turn on the brights, the low-beams also remain on.

What I think is happening is that I'm cooking the hotter high-performance bulbs by running the brights with the lows remaining on.  I love the extra brightness, but damn, expensive bulbs get expensive!

Anybody else notice this?

Yes - the "high performance" bulbs do have a shorter life span, the higher perfomance is achieved by designing the lamp filament to run at a lower voltage so that it is overdriven at the nominal 12V.

Regarding that "auto light" feature - there may be something incorrectly wired somewhere, it is supposed to do it's on/off magic through the #1 headlight relay, which is the same relay that switches the lights on from the manual switch, so the low beams & high beams should operate exactly the same no matter how the lights are turned on.

Regarding that "auto light" feature - there may be something incorrectly wired somewhere, it is supposed to do it's on/off magic through the #1 headlight relay, which is the same relay that switches the lights on from the manual switch, so the low beams & high beams should operate exactly the same no matter how the lights are turned on.

Hmmm.  I hadn't considered a miswiring possibility.  I'm 99+% sure I transferred all the wiring over faithfully as stock from the donor truck, but it also had an aftermarket remote starter (which rocks) installed.  I suppose that installation may have mildly boogered something up.  I can look into that.  Thanks for the suggestion.

this doesnt really fix your problem, but I recommend you buy "eurodezigns" headlights, I ran them in my focus since I bought it in 2002, one set, lasted 5 years, and they were VERY bright, I was/am pleased with them.

Best thing, if they do blow out, you put one bulb in a regular letter sized envelope and put duct tape around the sharp edges, put a money order for 6 bucks in it, a letter with your name on it, and they send you a new set.

You cant beat it, I would get some for my sammy, but theyre sealed beam.