a leak
Examples
The trouble with many men is that they have got just enough religion to make them miserable. If there is not joy in religion, you have got a leak in your religion.
If there's a leak out of my administration, I want to know who it is.
To the pilot of a deep sea submersible, upon finding out what would happen if the craft sprung a leak while submerged. I'll trust you to make sure that doesn't happen.
A CIA officer's name was blown and there was a leak and we needed to figure out how that happened, who did it, why, whether a crime was committed, whether we could prove it, whether we should prove it. Given national security was at stake, it was especially important that we find out accurate facts.
Don't take a leak on my shoes and tell me it's raining.