I've already tried applying it in so many pinch situations and it always saves me. For example is when I was still studying in college, I forgot my ID and the guard won't let me in, so I said I forgot my ID on the lab the other day, I've said that with a straight face and he let me in.
This is not an example of bluffing. Bluffing is an action when you are using your current situation in gambling (in a bad situation) to your advantage. Meaning, you will have the possibility to make your opponents to fold their cards by showing in your expression that you have a high card or the winning card. What you have given as an example is simply lying and not bluffing.