Never assume, always know.
I've been playing Metro 2033 and I'm about a third into a level where you're above ground, which requires you to wear a gas mask, and with that comes canisters. Now there was a market just before I got on this level selling canisters, and I assumed I had more than 1 canister remaining. Well, I didn't. So now I'm stuck on this level with a fucking dragon thing on top of me and Nazi's sniping my little cabin, with less than canister of oxygen left, and about 20 seconds of time you have without the gas mask on.
So now I have to restart the entire game, which held a few levels I'd rather remained finished.
Fuck.
Never assume, always know.
And when you scroll over the canisters, it even tells you how much you have left T.T
Anyway... that's a pretty sound piece of advice. Are you feeling OK?