well, a function will pretty much be some type of pattern.
something that doesn't behave in a quite predictable pattern
well, when it rains, the clouds come by and down you're wet, but is not as predicatable as we'd like.
the movement of stars.
eruption of volcanoes, very unpredictable.
downturns in the economy, check the 2008 market crash, many moving their heads both ways tried to see what happened, noone saw it coming, it just hit like a lightning.
there's another one, where a lightning strikes :).