Art teachers always say this. They’re always wrong.
The path of a raindrop
The membrane where two bubbles meet
The edge of a crystal
The surface of a frozen pond
? your turn
A beam of sunlight.
A strand of webbing under tension.
The place where a chainsaw went through a stump.
D’Oh! the sight that inspired this thread was a spiderweb, with perfectly straight spokes.
Crystals have straight edges.
Not if there’s any wind, and there often is.
The line that forms to purchase tickets for the latest broadway musical.
Oh, wait…dammit art teacher! You were right!
That’s not natural, unless there are packs of free roaming chainsaws somewhere.
And in that case, tell me where so I can AVOID that place. :eek:
. . . and I know these aren’t technically straight lines, but the back pattern of the Gaboon Viper is astonishingly rectangular for a reptile’s protective coloration.
Line of sight.
Tree leaves have lots of straight lines.
Not straight, but rather a segment of a (quite large) sphere. 
A comedy sketch.
Sadly, both are distorted by gravity.
There are no straight lines. They all experimented in college. In gaol they’d have bitches. It’s a continuum, man.
I never heard an art teacher say that.
It’s also going to be jagged at some magnification.
Similarly, the edges in a snowflake.
A plumbline (of different sorts).
The horizon at sea, more or less.
None of these are truly straight lines at all. They may appear straight, but after magnification or true measurement, you’d see they are not straight at all, and usually far from it.