An idiom meaning "The ideal form of something always seems perfect, but in reality it probably isn't"
I always use the magnificently descriptive phrase I first encountered in Jonathan Lethem's brilliant book Girl In Landscape – "The map is not the territory."
The line may look straight, but the road goes straight up the mountain, or there's a crevasse, or it was a computer approximation, or you die of a heart attack before you get there.
A masterpiece in six words; stunning.
It was coined by Alford Korzybski.