Commuting today, I noticed that the car behind me had LED headlights that appeared to "bobble" within their lenses--in other words the reflection of the car and the reflection of the headlight lamps moved in opposite directions.
The illusion appeared only in my rear view mirror and not in my wing mirrors. When I held the rear view so it could not shake the illusion disappeared.
Since it had to do with the vibration of the rear view mirror I could only think of two things: some sort of path-length difference effect that is not the same for the car's image as for the LEDs' images, or some sort of frequency difference between the vibration of the rear view mirror and the LED lamps. (I always think of LEDs as being DC but I don't know how they are implemented in cars.)
Has anyone else noticed this illusion? Does anyone else have an explanation?