I would probably go with option (5.) Leave the half-cup in the pot and start a new pot with just a little less water.
No sense wasting it, as long as it’s relatively fresh. If it was hours-old, I’d dump it and start a new pot.
I wouldn’t pour it into the decaf, because some people are very sensitive to caffeine. I certainly wouldn’t just leave it sitting on a hot burner, and I’d never walk away from an empty coffee pot in an office setting. I’d always make a fresh pot.
When I waitressed, back in the stone age, our mantra was decaf into regular, never pour regular into decaf. I can just imagine some litigation happy schmuck seeing regular poured into decaf and inventing a heart condition that means caffeine=certain death, settlement city.
I’d likely slurp up half my cup and empty the pot into it, then start a new one.
Well, there’s no need to invent a heart condition that can mean “too much caffeine = ventricular tachycardia.” Try RVOT-VT (right ventricular outflow tract ventricular tachycardia.)
I’m just trying to understand why the person who could end up with a trip to the ER because the restaurant lied about their product being caffeine free would be labelled the “schmuck” rather than the person who assaulted them with a potentially hazardous substance being labelled a thoughtless [obscene name.]
[sub]And, yes, I’m aware that decaffeinated coffee does contain a small amount of caffeine. I can handle the amount of caffeine in an occasional cup of a decaff sissy coffee drink, or in a small amount of chocolate, but I don’t push my luck by having them at the same time. Due to the fact I realize that the world is full of thoughtless schmucks, though, I’ll only drink “decaff” that I make myself, or purchased from a place where I can quiz the person who poured it and make sure they know it’s important for it to be decaff.[/sub]
So, to get back to the original question:
Please, DON’T mix the caffeinated coffee into the decaf. It’s unethical to screw around with other people’s health.
I would never put regular into the decaf. I may put decaf into the regular depending on how much of each there was (if there’s only a little decaf and a lot of regular, it shouldn’t change the tast much at least for our nasty office coffee).
However, it’s never really been an issue. We have four burners and usually two pots of regular and one pot of decaf going at any given time. So I can put the leftover regular into another regular pot. I don’t drink the decaf so I don’t know what people do if that gets low (they probably put it in the regular pots).