I have made a few impulse buys of firearms and I'm not sure I want to sit down and take a long hard look at my success rate.
But for the most part, I put a whole heckuva lot of time, thought, research and patience in to my gun buys.
Ammo? I only buy rimfire. Even then, I'm partial to a few different very specific brands and I stick to them. I roll all my own centerfire. So to answer the "spirit" of the question, I've made a bunch of "impulse" buys on new powders I've never tried and no bullet with a low price is a bullet I won't give a twirl.