Originally Posted by noderaser
What does the base of the bulb look like? Perhaps it wasn't machined right, and had a larger area on the screw or tip, causing the short?
It doesn't look like the tip would be the problem. From the looks of the damage, the screw part of the bulb extended down to where the ring of the 3-way bulbs are, and the 2nd hot terminal reached it just barely. Most of the arc damage is at that 2nd hot terminal. There's a bit of screw shell damage. What Ann Brush said seems to be the cause.