We're programmers. Overheard snatch of conversation between co-worker and boss (cleaned up):

> Yes, we can certainly look into this new technology, but who knows what **reefs** await us?

After the call ended, we quizzed him on his use of `reef` in this way. He said that in Russian it is used as an idiom to represent potential hidden problems that are calamitous if encountered (pretty apt in the context, actually). I couldn't think of any English equivalent -- can you?