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Also, the dual-filament bulbs have one bayonet pin close to the end of the base, and one further up (nearer the glass envelope). This is to insure the correct polarity (brake = high wattage filament, tail = low wattage filament).
However, if you try hard enough, you can deform the socket enough to put the bulb in backwards.
If you don't try hard enough, you will not seat the bulb completely.