[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
Unfortunately, there's nothing that says the tea will force your lunch-mate to drink on the first thing you think about that would cause him to choke. You could run through a dozen true and shocking guesses in your head without him feeling any urge to drink. Once you get bored and give up thinking of new hypotheses, if your lunch-mate hasn't drunk from the can, the vendor's guarantee is still intact because none of the the tea has been drunk. Why does this remind me of the halting problem?
On the other hand, if you wait until after your lunch-mate has taken his first sip (taking the risk that something unexpected and shocking will happen when he does so), and you have resolved to take away the drink immediately after his second sip, you might be in a better position.
You also need to hope that the shocking event that causes your companion to choke is not an epiphany on his part where he suddenly deduces one of your secrets.