Thought I'd make fun of myself here.
Had a 98 S70 forever (close to 300k). Replaced a gazillion of headlight bulbs. Have had a 05 S80 since new, - replaced 1/2 a gazillion headlight bulbs . (The XC90 didn't stay long enough for me to have to do it - but the dealer did).

Always - repeat always - struggled with the damned things. First is that little cap, comes off easy but man, never screws on easy. But the killer is that bulb. I have lined up that little tab with the track on the bottom of the socket and jammed, twisted, forced, cajoled, swore and a few other things before I could get the clip back on. I get kinda dizzy when I lean over to do the job which makes it worse. More than a few hours (in 10 to 20 minute increments) have been spent in front of an auto parts store battling the little beasts in the dark.And my light pattern sucks big time.
Last night I had to do the right low beam. Again. Got my new Silverstar Ultra and headed out to the garage. Took a new flashlight for my wife's center console as well (to replace the corroded battery model she had) .
Popped the cap, Couldn't budge tjhe bulb. Got the curved needlenose pliers out, pulled that SOB outta ther. It was - as always- so dark I was feeling my way around when I remembered the flashlight in my pocket. Turned it on, bent over and - FOR THE FIRST TIME - saw the notch in the top of the socket. Placed bulb in "upside down.". Dropped right in, so easy. Hmmm. But black thingy inside housing was all crooked from years of forcing bulbs in the wrong way. Straightend it up buttened the light back up . Turned on headlight and examined headlight patter on front wall of garage. Hmmm. Took other side out, bent black thingy to correct orientaion, installed buld "upside down" an, closed it up and turned on lights again. Beautiful!!.

So embarrassed. But at least now I know.