Nuke in the sand

A very big, very dangerous or concerning looming threat or danger. It doesn't have to be a large scale danger (like a war) or involve literal nukes, but could also be something that could involve severe emotional devastation or an unrecoverable argument between friends (Like mentioning a topic that could give someone a major panic/anxiety attack).

Person A: Why don't you talk about (game) in this chat?
Person B: Well, I am a fan of that game, but one of my friends has a really big anxiety trigger involving it, so I don't mention it out of respect.
Person A: Thank God you told me, that could've been a nuke in the sand.