Back in my days doing Imaging and Video engineering at Sun, we had a joke about the three global television systems: NTSC, SECAM and PAL. NTSC, we claimed, stood for Never Twice Same Color. SECAM, a French system, was Something Essentially Contradictory to American Methods, and PAL coming from the UK was Peace At Last.
Well, every time I try to do landscape photography I run into nature's equivalent of NTSC, which is NTSL (Never Twice Same Light.) Any photography with naturally available light (ie, not using flash or artificial light) is subject to NTSL, but since landscapes depend so utterly on the lighting, it becomes crucial to pay attention to the light.
One can wax philosophical about this whole notion. I mean, isn't life a long series of never-twice experiences? The great Bengali poet Tagore got it - like many other mysteries of life - and expressed the idea beautifully:
Aar ki kokhono kobey
Emoni shondha hobey?
Jonomer moto, hai, hoye gelo hara
Will there ever be another
Twilight like this one?
Gone for good... (Bengalis forgive me for butchered translation)
Back to photography... I was wandering looking for a good picture when the ground turned velvety due to some funny clouds. I knew it would not last, and took a picture right away.

About a minute later I had set up my tripod and composed a better organized scene. But by then the light was... never twice!

PS, the NTSC/PAL/SECAM jokes originally appeared in Anil Jain's Fundamentals of Digital Image Processing.