One known catch22 situation- is an observation car which has the marker lights which are typically 1.5V incandescent grain of wheat lamps.
They used diodes as voltage regulators to create and limit the approximate 1.5V for the bulbs- but that only works with incandescent main light load. When you slap in LED bulbs, then the marker lights won't light because we are not carrying enough current to push the limiting diodes into the knee curve of the voltage drop- creating that critical 1.5V source.
