Help find an idiom

I was asked for a translation for a Russian saying/idiom: "подводный камень", which literally means "underwater stone". Admittedly I first had to ask what it means. So what's an English saying for "unforeseen challenges" or "hidden dangers". For inspiration (thank you Chrysalis):


Suggestions so far:

Pretty close:

Chrysalis: Bumps in the road
me: That's close but not really unforeseen, I don't think.
Chrysalis: Unforeseen bumps in the road?

Also from my conversation with Chrysalis:

Getting thrown a curve ball.

And ever helpful:

Well, landmines are sort of that... -Tommy C