One Light or Two?

A very strange thing happened to me today.

I had just pulled over to the side of the road in a shopping centre car park when a woman in her thirties or forties - clearly a driver judging by the keys in her hand - came up to me from behind the car and notified me that one of my rear lights wasn't working.

"One of the brake lights?" I asked.
"I don't know," she said.
"Which side?" I asked, pressing on the brake pedal.
"Not those: one of the middle ones," she said.

So I got out of the car to see what she meant.

"That one," she said, pointing to my left indicator light. "You see it's not blinking?"
"That's because I'm indicating right," I replied a little perplexed.
"But shouldn't they both be blinking?" she asked.
"No. Is this what you mean?" I said, getting back into my car and switching the hazard warning lights on.
"Yes! Now they're both working again!" she cried.
"If only one light is on, it's the left or right indicator," I explained.
"I never knew it could do that," she replied, walking away towards her car, clearly amazed at the revelation.