I'm not sure what exception you are referring to, but I would guess it has to do with light, since light gets weirder and weirder the more you look at it. So either it has to do with light's properties changing in different conditions (acting like a wave sometimes and like ??? (yikes, my poor memory is blocking on a simple word, help, Alzeimer's runs in my family!), or it has to do with the speed of light which, if I recall correctly, speeds up no matter which side of 186,000m/sec it is on.