Bulbs:444 vs. 555
Pinball bulbs suffer form vibration sickness, what with tha ball bouncing around. Mostly, they live through this but some high-vibration areas cause trouble. the filament gets little shorts in its spirals, which make the filament carry more current, which makes it hotter and brighter - until it burns up. And as it burns hotter, it can cause insert, lens or plastic damage.
The classic trouble spot is the pop bumper. The metal socket legs take up some of the load - unless the body of the bumper is a bit loose, or the legs or bulb are touching something. But they can't stop all of it. Also, some playfield locations may also have more vibration because of flipper motion or ball strikes.
The easiest solution is to use a 444 bulb in the problem locations. These bulbs have an extra supporting wire for the filament, which makes them much more robust. I get mine from Rick at BAA - haven't seen them anywhere else. At $0.40 each, they are a bit too expensive to put all through a machine, but a great replacement for sensitive locations. And they should be in all pop bumpers.
Update: I have since received the Eiko "rough usage" #259 bulb. Marco claims that these bulbs have "Same structural durability and voltage as the #444.". I am skeptical of this claim because these bulbs do not have the third filament support.
The 444 is on the left, and the 555 in the middle. The 259 is on the right. Click for a really big picture.
Here are the filaments, exposed by a bit of brutality on the glass. The numbers at the top are the thickness that I measured of the wire. I didn't measure the resistance, as I didn't have my good meter to hand. Measuring length was not practical. The filament does not straighten easily. You can see here that the 259 looks better assembled than the 555. Looks like it also has a longer filament. but it is missing that third support, which I believe to be critical for vibration resistance. I have a friend with a LoTR that eats even 444s, I'll have to try them in that and get a log as to how long each lasts.