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?