A matter with the Comed-Tea that was bugging me for a while:

Chapter 14:

SO THAT'S HOW THE COMED-TEA WORKS! Of course! The spell doesn't force funny events to happen, it just makes you feel an impulse to drink right before funny things are going to happen anyway!

Hypotheses: Comed-Tea on person = impulse to drink, Comed-Tea not on person = no impulse to drink.

According to Chapter 12:

Harry couldn't help but feel the urge to drink another Comed-Tea. (And when he didn't...) Harry inhaled his own saliva and went into a coughing fit just as all eyes turned toward him.

So no matter what, even if you don't end up drinking it, you will get the Impulse before something funny happens.

Chapter 46:

I have been saving them for special occasions; there is a minor enchantment on them to ensure they are drunk at the right time. This is the last of my supply, but I do not think there will ever come a finer occasion.

So Harry has used up all of his Comed-Tea. (edit: it appears that Harry actually has tons left unless he's not mentioning some he drank/gave away, look at bottom of post)

...

WHY? WHYYY?!

It is apparent that you'll still get the impulse to drink whether or not you do end up drinking. So why didn't he save a can he's never ever going to drink?

Even if Harry will end up choking on his saliva, wouldn't the early notification of something ridiculous happening be helpful to him in any way? Like... it'd be an early warning to be prepared for whatever another person could say/do in conversation. Or if he's looking for interesting information, say from the library, he can just walk by all the shelves until he gets the Impulse-- that'd be an indicator that he's near the shelf that has the interesting book. There might be more uses.

Chapter 14:

Thankfully, Harry’s panicking brain remembered at this point that he did have something he’d been planning to discuss with Professor McGonagall. Something important and well worth her time. It was at this point that Harry realized he was faced with a priceless and possibly irreplaceable opportunity to offer Professor McGonagall a Comed-Tea and he couldn’t believe he was seriously thinking that and it would be fine the soda would vanish after a few seconds and he told that part of himself to shut up.

The charm even works for other people. If, for example, Harry wishes to test whether or not someone knows that Voldemort is alive, he could see if he has the Impulse to give that person a drink, all while thinking about saying that "The Dark Lord is still alive". If he gets the Impulse, they don't know. If he doesn't, then they already know/has been suspecting that he's been alive.

Chapter 8:

The boy reached into his pouch and said, "can of soda", retrieving a bright green cylinder. He held it out to her and said, "Can I offer you something to drink?"

Hermione politely accepted the soda. In fact she was feeling sort of thirsty by now.

In fact, just asking, "Are you feeling thirsty?" seems to be enough to trigger the charm's apparent spit-taking powers. Harry could think about talking about Voldemort, and ask if the other person's thirsty. If yes, they would take whatever he's going to say as a surprise, if no, then they won't. Geebus this thing is powerful.

edit: actually, I'm going to check the text and see Harry actually used up his supply. Be right back.

Chapter 7: “Two dozen cans please.” (24) He tossed a can to Draco and then started feeding his pouch... (23) (Harry's drinking one too) (22) Harry snarled, threw the can violently into a nearby garbage can, and talked back over to the vendor. “One copy of The Quibbler, please.” He paid over four more Knuts, retrieved another can of Comed-Tea from his pouch... (21)

Chapter 8: The boy reached into his pouch and said, “can of soda”, retrieving a bright green cylinder. He held it out to her and said, “Can I offer you something to drink?” (20)

Chapter 12: Harry reached into his pouch and whispered, “Comed-Tea”. (19)

Chapter 46: “Three sodas." (16)

Nevermind, Harry lied, he still has tons unless he's been drinking them and not mentioning it. However the Comed-Tea hasn't been mentioned since, so it might actually be all gone.