What is the last problem you found out about only after it cost you something?

I used to find out about problems in my business only after they had already become expensive. A client who was ready to leave before they said anything. A hire who had checked out weeks before handing in notice. A number that had been sliding while I was buried in other things. None of the warnings were hidden, I simply was not looking in the right place at the right time.

What is the last thing your business tried to tell you before you actually heard it, and how much did the delay end up costing you?