when you do something to your own detriment - not an accident like falling down the stairs, it's more like getting yourself in trouble or making things difficult for yourself. for instance, while talking you might accidentally reveal something you'd done wrong.

Shoot yourself in the foot is a phrase that comes from a time in the US when nearly everybody carried a side arm (gun). If you went to draw your gun from the holster, and you were not careful, you could accidently fire the gun too early and shoot yourself in the foot. If you did that, you had harmed yourself before your could use the gun to defend yourself and had been your own worst enemy. This phrase has lived on and is now used anytime somebody causes themselves a problem.

to do or say something that will cause you a lot of trouble or harm, especially when you are trying to get an advantage for yourself Eg) shoot yourself in the foot when you play game all day.

Imagine you get excited and you want to pull your gun and shoot at somebody or something. But you get careless and shoot the gun while it is still in the holster. You shoot yourself in the foot. Just take this image and apply it to your context.